Transition build system to CMake

This pull request changes the build system for OpenMC to CMake instead of GNU make. The way that options are chosen is slightly different. Rather than running `make OPENMP=yes`, you would now need to run `cmake -Dopenmp=on .` followed by `make`. By default, running `make` in the src/ directory will create a build/ directory and run cmake from there for an out-of-source build. Another difference is that you do not need to specify MPI=yes as an option. Instead, CMakeLists.txt will check whether your Fortran compiler (set by the FC environment variable) looks like an mpi wrapper and if so, MPI is turned on.

There are a still a number of things to be done before this pull request is complete:
- [x] Compilation flags for different compilers
- [x] Ability to compile with MPI
- [x] Ability to compile with OpenMP
- [ ] Ability to compile with HDF5
- [ ] Ability to compile with PETSc
- [ ] Installation of executables/scripts/man pages
- [ ] Update documentation