CMake Improvements

[mondus]

We should add some CMake flags to improve the build experience this should include

• Examples: A flag for each to enable building
• Documentation: We should remove the dependencies for documentation building if only building the library
• Testing: We should remove the dependencies for testing if only building the library

[jatkinson1000]

@SimonClifford has pointed out that we do not quite match the 'ideal' cmake installation structure meaning that we have to 'overspecify' directories when linking etc. This can be fixed and the CMake flags and commands made simpler.

[jatkinson1000]

We might also tie this in with #9 (or at least be aware of it) seeking to add CUDA as a language to CMake to make building with GPU an option.

[jatkinson1000]

@ElliottKasoar is this something you might fancy taking a look at after #38 ?