BUG - (Windows) Absolute path not correctly recognized as such in aas_desc_outputs

[AljenU]

Determining whether a path is an absolute path is currently mostly done by checking whether the first character of the path is a forward slash. On windows, this is not a correct way for identifying an absolute path. There, such a path starts with a drive letter.

Add a function to determine whether a path is absolute or not.

Notes:
To be very safe, would need to go to something like https://nl.mathworks.com/matlabcentral/fileexchange/28249-getfullpath
For now, something inspired by https://svn.oss.deltares.nl/repos/openearthtools/trunk/matlab/general/io_fun/abspath.m will probably do good enough