Add option to specify MANTIDPATH to linting script.

Refs #9951
Select directories to run pylint through cmake.

More consistent with how cppcheck is run.
Refs #9951
Allow a list of excluded paths for pylint.

Refs #9951
Turn off warning about undefined variable

This mostly comes up with functions from mantid.simpleapi because they
are generated and not static.
Refs #9951