Add --enable-fortran option

[CamStan]

Require users to be proactive when they want to use fortran since we conflict with certain fortran compilers (#300, #304). This makes it easier to protect the user when installing with spack, especially with an incompatible fortran compiler.

Fixes: #313

Description

• configure.ac: Added --enable-fortran and only calls AC_PROG_FC and sets up mpif when enabled
• client/src/Makefile.am: only include library and header when fortran enabled
• examples/src/Makefile.am: only build fortran examples when fortran enabled
• docs/api-mount.rst: added note for users to include the fortran option if they want fortran

How Has This Been Tested?

Manually built with and without --enable-fortran and verified the associated files were included/excluded respectively.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the UnifyCR code style requirements.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.
• All commit messages are properly formatted.