Added path suffix to search for scotch.h.
This MR improves the ability for CMake to find an available version of Scotch.
- It adds support for a couple of environmental variables to help find scotch in non-system paths.
- It adds a suffix path to the ThirdParty Scotch library search to allow it to find the header file on some Linux distributions where it is located within a sub-directory.
- It moves the finding of the Scotch libraries out into a FindScotch module.
- Adds support to check the version of Scotch and select what is acceptable.