feat(types): Add type hints and code optimizations

Python3.8 is almost end-of-life and backwards compatibility
to versions before type hints is not a priority anymore.

This commit is a start and by no means complete or correct
for all instances.

Furthermore I included code optimizations. These mainly regard
raising of exceptions, pytest style tests and some minor
refactoring.

pyproject.toml

@@ -7,4 +7,4 @@ requires = [
 ]
 
 [tool.setuptools_scm]
-fallback_version = "0.3.8"
+fallback_version = "0.3.8"

salem/__init__.py

@@ -1,26 +1,22 @@
-"""
-Salem package
-"""
-from __future__ import division
+"""Salem package."""
 
-from os import path
-from os import makedirs
 import sys
+from collections.abc import Callable
 from functools import wraps
+from pathlib import Path
 
 import pyproj
 
 from .version import __version__
 
 
-def lazy_property(fn):
-    """Decorator that makes a property lazy-evaluated."""
-
+def lazy_property(fn: Callable) -> Callable:
+    """Lazy-evaluate a property (Decorator)."""
     attr_name = '_lazy_' + fn.__name__
 
     @property
     @wraps(fn)
-    def _lazy_property(self):
+    def _lazy_property(self: object) -> object:
         if not hasattr(self, attr_name):
             setattr(self, attr_name, fn(self))
         return getattr(self, attr_name)
@@ -32,44 +28,71 @@ def _lazy_property(self):
 wgs84 = pyproj.Proj(proj='latlong', datum='WGS84')
 
 # Path to the cache directory
-cache_dir = path.join(path.expanduser('~'), '.salem_cache')
-if not path.exists(cache_dir):
-    makedirs(cache_dir)
-download_dir = path.join(cache_dir, 'downloads')
-if not path.exists(download_dir):
-    makedirs(download_dir)
+cache_dir = Path.home() / '.salem_cache'
+cache_dir.mkdir(exist_ok=True)
+download_dir = cache_dir / 'downloads'
+download_dir.mkdir(exist_ok=True)
 
 sample_data_gh_commit = '454bf696324000d198f574a1bf5bc56e3e489051'
-sample_data_dir = path.join(cache_dir, 'salem-sample-data-' +
-                            sample_data_gh_commit)
+sample_data_dir = cache_dir / f'salem-sample-data-{sample_data_gh_commit}'
 
 # python version
 python_version = 'py3'
 if sys.version_info.major == 2:
     python_version = 'py2'
 
 # API
-from salem.gis import *
-from salem.datasets import *
-from salem.sio import read_shapefile, read_shapefile_to_grid, grid_from_dataset
-from salem.sio import (open_xr_dataset, open_metum_dataset,
-                       open_wrf_dataset, open_mf_wrf_dataset)
-from salem.sio import DataArrayAccessor, DatasetAccessor
+from salem.datasets import (
+    WRF,
+    EsriITMIX,
+    GeoDataset,
+    GeoNetcdf,
+    GeoTiff,
+    GoogleCenterMap,
+    GoogleVisibleMap,
+)
+from salem.gis import (
+    Grid,
+    check_crs,
+    googlestatic_mercator_grid,
+    mercator_grid,
+    proj_is_latlong,
+    proj_is_same,
+    proj_to_cartopy,
+    transform_geometry,
+    transform_geopandas,
+    transform_proj,
+)
+from salem.sio import (
+    DataArrayAccessor,
+    DatasetAccessor,
+    grid_from_dataset,
+    open_metum_dataset,
+    open_mf_wrf_dataset,
+    open_wrf_dataset,
+    open_xr_dataset,
+    read_shapefile,
+    read_shapefile_to_grid,
+)
 from salem.utils import get_demo_file, reduce
 
 try:
-    from salem.graphics import get_cmap, DataLevels, Map
+    from salem.graphics import DataLevels, Map, get_cmap
 except ImportError as err:
-        if 'matplotlib' not in str(err):
-            raise
+    if 'matplotlib' not in str(err):
+        raise
+
+    def get_cmap() -> None:
+        msg = 'requires matplotlib'
+        raise ImportError(msg)
 
-        def get_cmap():
-            raise ImportError('requires matplotlib')
+    def DataLevels() -> None:
+        msg = 'requires matplotlib'
+        raise ImportError(msg)
 
-        def DataLevels():
-            raise ImportError('requires matplotlib')
+    def Map() -> None:
+        msg = 'requires matplotlib'
+        raise ImportError(msg)
 
-        def Map():
-            raise ImportError('requires matplotlib')
 
 from salem.wrftools import geogrid_simulator