Issue272 + Issue265 mpi and main in separate process

[mikebentley15]

Fixes #265 and fixes #272

Description:
Allow calling a main()-like function from a FLiT test. Also, be able to call a main()-like function that uses MPI from a FLiT test.

The main()-like functionality is the following:

• Done with two components:
  1. REGISTER_FLIT_MAIN() that takes the function pointer, and
  2. call_main() that takes the function pointer and command-line arguments
• The main()-like function is called from a child process (that is the same executable, but with special flags)

The MPI functionality is the following:

• Remove all MPI functionality from before (except for the compiling with MPI)
• When FLiT tests are called, they are not under MPI
• Instead, the user calls call_mpi_main() from the test where they specify everything the call_main() function requires plus an mpirun command with flags.
• The new subprocess(es) is(are) now launched with the user's mpirun command and everything just works.
• The downside is that tests using MPI must use a main()-like function. There is no other way. I think this is a reasonable downside, and worth the exxtra stability our current approach affords
• Add utility functions in src/subprocess.cpp

Documentation:

• Remove documentation to removed MPI functionality
• Add many sections to "Writing Test Cases" page describing how to write a test that wraps around a main() function and how to write MPI tests with examples.

Tests:

• Update the MPI tests
• Test everything in src/subprocess.cpp
• Add unit tests for call_main()
• Add tests for the TestFactory class
• Add tests for the registerTest() function used by the REGISTER_TEST() macro