Bug when project is saved on another drive than GWHAT installation

[jnsebgosselin]

What steps will reproduce the problem?

1. Start GWHAT from drive C:
2. Try to open a project saved on drive D: (or any other letter)

Please provide any additional information below

Saving WHAT preferences to file...
Traceback (most recent call last):
  File "C:\Users\User\gwhat\gwhat\mainwindow.py", line 248, in new_project_loaded
    self.whatPref.save_pref_file()
  File "C:\Users\User\gwhat\gwhat\mainwindow.py", line 310, in save_pref_file
    fcontent = [['Project File:', os.path.relpath(self.projectfile)],
  File "C:\Anaconda3\envs\spyder-dev\lib\ntpath.py", line 562, in relpath
    path_drive, start_drive))
ValueError: path is on mount 'D:', start on mount 'C:'