BUG - (Windows) Comparing paths can fail on non-native filesep in aa_init

[AljenU]

Related to #242

The native file separator on Windows is '\', as also returned by the matlab filesep function. However, paths that use '/' as a file separator, or even a combination of forward and backward, are also accepted by Windows as valid path specifiers.
Any paths returned from functions will generally have the native file separator.

Thus, doing e.g.
addpath('C:/Users\username/test')
will not give an error. Afterwards, the first entry in path() will show to be
C:\Users\username\test;

As a result, user provided paths should be checked and/or preprocessed when they are going to be used to compare with code provided paths.

TBD: where and how to do that so it is as failsafe as possible.