Implement compatibility fixes for MacOSX and WSL Blocks #411 and #412"

Putting functionality for resource handling in _platform.py and replacing os.sched_getaffinity() with a globably compatible command

Changes proposed in this merge request:

*  Stopped safe_remove attempting to unlink dirs
    Previously safe_remove would attempt to unlink a path
    Before attempting to remove it if it was a dir
    Now it checks for a dir before that step

*  Adding darwin.py platform
   Adding functionality to recognise Darwin as a platform in plaform.py

*  Move Cas call to Platform

*  Moving max-jobs call to Platform

*  Add FUSE check to linux.py
    This needed to be implemented as Windows WSL
    Does not currently support FUSE

*  Moved `resource.getrlimit()`
    Removed `resource.getrlimit()` functionality from app.py
    Added `resource.getrlimit()` functionality to platform.py as function
    Called new function from __init__.py of linux.py and unix.py

This merge request, when approved, will close issue/bug: #575 (closed), #411 (closed) and #412 (closed)