Fix path handling in SANS isis_reductions_steps

[martyngigg]

The path handling in scripts/SANS/isis_reduction_steps.py is inconsistent. Some places use os.path.join and others simply add / separators. These all should harmonize on using os.path.join.

Another related problem is path handling when loading the pixel correction file on line 1581 of scripts/SANS/isis_reduction_steps.py. The load is done via an ```eval}} call where all of the code is in a string. If the self._pixel_file is a windows path that happens to contain a directory name that starts with a character that can be interpreted as an escape character, e.g.

C:\release\Data\LOQ/FLAT_CELL.061

where the \r would be interpreted as a carriage return then the path is invalid and the command fails. In this particular case I think line 1581 just needs to be changed to

load_com = self._load+'(Filename=r"'+self._pixel_file+'",OutputWorkspace="'+pixel_adj+'"'

to make the Filename string a raw string so that escape characters are ignored.

[martyngigg]

This issue was originally trac ticket 9873