Custom Model Importer won't read a filename with escaped spaces

[jbudynk]

On OS X:

The 'install' button is disabled when a filename like

/Path/to\ my/model.txt

is used. The 'install' button is enabled when the "\" is omitted, but on trying to fit with this model, the fit fails with "No such file or directory: '/Path/to\\'".

The Custom Model Importer should either accept "\" in the filename, or append "\" to spaces when loading the model to fit in Sherpa.

On Linux, spaces are allowed in the file path. "\" still aren't allowed in the path. There's no problem with reading paths/filenames with spaces on Linux.