Additional CMake changes

[SeanBryan51]

See #200 for the initial implementation of a CMake based build system. This issue documents changes that should follow after this pull request:

• List source files in alphabetical order in CMakeLists.txt. The order of source files was chosen so that we could demonstrate binary equivalence in executables between the CMake build and the Makefile build.
• Use find_package(MPI REQUIRED) instead of specifying mpif90 when invoking cmake for MPI case. This is done so that
  1. We can create separate targets for the serial and MPI executable and only link against the MPI libraries when needed without relying on the MPI compiler wrapper. This lets us compile all executables (serial and parallel) with a single invocation of CMake.
  2. We can use object libraries to compile the object files common to
     both serial and MPI builds. This saves compiling these object files for
     each executable (serial and MPI), reducing compilation time.
• The previous task also lets us remove the MPI option from build script. The same can be achieved by specifying --target cable-mpi when invoking CMake.

Originally posted by @SeanBryan51 in #200 (comment)