PR: Fix missing hidden imports for PyInstaller packaging

[jnsebgosselin]

This is a follow up of PR #249.

When trying to start GWHAT with the package generated by PyInstaller, we get the following error traceback:

Starting GWHAT...
Traceback (most recent call last):
  File "gwhat\mainwindow.py", line 20, in <module>
  File "C:\Python36-x64\lib\site-packages\PyInstaller\loader\pyimod03_importers.py", line 627, in exec_module
  File "site-packages\matplotlib\__init__.py", line 141, in <module>
  File "C:\Python36-x64\lib\site-packages\PyInstaller\loader\pyimod03_importers.py", line 627, in exec_module
  File "site-packages\matplotlib\cbook\__init__.py", line 33, in <module>
  File "C:\Python36-x64\lib\site-packages\PyInstaller\loader\pyimod03_importers.py", line 627, in exec_module
  File "site-packages\numpy\__init__.py", line 151, in <module>
  File "C:\Python36-x64\lib\site-packages\PyInstaller\loader\pyimod03_importers.py", line 627, in exec_module
  File "site-packages\numpy\ctypeslib.py", line 369, in <module>
  File "site-packages\numpy\ctypeslib.py", line 358, in _get_typecodes
  File "site-packages\numpy\ctypeslib.py", line 358, in <dictcomp>
ModuleNotFoundError: No module named 'numpy.core._dtype_ctypes'
[18312] Failed to execute script mainwindow