Refs #9873 - Change to os.path.join.
Refs #9873 - Remove use of eval().

The comments in the previous code indicated that CalculateNormISIS could
be inherited from and that base classes could potentially want to override
_load and _load_params.  This has not been done anywhere in the code, and
I cannot see a ticket that would require it, so I'm just removing the
complex string concatenation and replacing it with a simple call to
LoadRKH, which is the algorithm used to load pixel correction files.
Merge branch 'bugfix/9873_path_handling_in_isis_reduction_steps' into develop