Add check before setting multiprocessing context to prevent the Runti…

…meError (#4620)

* Add check before setting multiprocessing context to prevent the runtime error: context has already been set

When `pycbc` is used with some multiprocessing package such as `dask`, importing `pycbc` will cause setting the multiprocessing context repeatedly. The `RuntimeError: context has already been set` will happen. This fix will check the context before setting it.

* Update __init__.py

Change raising error to warning

* wrap the lines

---------

Co-authored-by: Yumeng Xu <yumeng.xu@physik.uzh.ch>

pycbc/__init__.py

@@ -212,8 +212,13 @@ def makedir(path):
     # preserve common state information which we have relied on when using
     # multiprocessing based pools.
     import multiprocessing
-    if hasattr(multiprocessing, 'set_start_method'):
-        multiprocessing.set_start_method('fork')
+    if multiprocessing.get_start_method(allow_none=True) is None:
+        if hasattr(multiprocessing, 'set_start_method'):
+            multiprocessing.set_start_method('fork')
+    elif multiprocessing.get_start_method() != 'fork':
+        warnings.warn("PyCBC requires the use of the 'fork' start method"
+                      " for multiprocessing, it is currently set to {}"
+                      .format(multiprocessing.get_start_method()))
 else:
     HAVE_OMP = True