gui: Remember the path to the input files

When a user navigates to a directory to read the YML file,
then wants to open a geqdsk file, start in the same directory.

hypnotoad/gui/gui.py

@@ -80,6 +80,9 @@ def __init__(self):
         super().__init__(None)
         self.setupUi(self)
 
+        # Used in file dialogs
+        self._current_dir = "."
+
         try:
             self.menu_File.setToolTipsVisible(True)
             self.menu_Mesh.setToolTipsVisible(True)
@@ -452,7 +455,7 @@ def select_options_file(self):
         """Choose a Hypnotoad options file to load"""
 
         filename, _ = QFileDialog.getOpenFileName(
-            self, "Open options file", ".", filter=YAML_FILTER
+            self, "Open options file", self._current_dir, filter=YAML_FILTER
         )
 
         if (filename is None) or (filename == ""):
@@ -461,6 +464,9 @@ def select_options_file(self):
             self.write("Could not find " + filename)
             return
 
+        # Record the directory so user doesn't have to navigate again
+        self._current_dir = os.path.dirname(filename)
+
         self.options_file_line_edit.setText(filename)
         self.filename = filename
         self.read_options()
@@ -506,7 +512,9 @@ def read_options(self):
     def select_geqdsk_file(self):
         """Choose a "geqdsk" equilibrium file to open"""
 
-        filename, _ = QFileDialog.getOpenFileName(self, "Open geqdsk file", ".")
+        filename, _ = QFileDialog.getOpenFileName(
+            self, "Open geqdsk file", self._current_dir
+        )
 
         if (filename is None) or (filename == ""):
             return  # Cancelled
@@ -517,6 +525,9 @@ def select_geqdsk_file(self):
             )
             return
 
+        # Record the directory so user doesn't have to navigate again
+        self._current_dir = os.path.dirname(filename)
+
         self.geqdsk_file_line_edit.setText(filename)
         self.geqdsk_file_line_edit.setStyleSheet("")
 
@@ -643,7 +654,7 @@ def write_grid(self):
         filename, _ = QFileDialog.getSaveFileName(
             self,
             "Save grid to file",
-            self.gui_options["grid_file"],
+            os.path.join(self._current_dir, self.gui_options["grid_file"]),
             filter=NETCDF_FILTER,
         )