Enforce forward slashes as path separators

[belluzj]

#22

[anthrotype]

lgtm

[belluzj]

If the implementation in MutatorMath is validated, I should maybe also use it in this project's code, for example here?

[belluzj]

I have added some code but I couldn't run the tests locally, because some test files are missing:

λ python Lib\designSpaceDocument\__init__.py
Traceback (most recent call last):
  File "Lib\designSpaceDocument\__init__.py", line 2179, in <module>
    __removeAxesFromDesignSpace(p)
  File "Lib\designSpaceDocument\__init__.py", line 1313, in __removeAxesFromDesignSpace
    f = open(path, 'r')
FileNotFoundError: [Errno 2] No such file or directory: 'testCheck.designspace'

[anthrotype]

@LettError can we get this merged?