Integrate GAPD

[CFGrote]

Integrate GAPD into simex_platform:

• [x] Add GAPD to the cmake build system
• [ ] Add data converters to IOUtilities:
  • [ ] Input from MD format(s)
  • [ ] Output to singFEL/crystFEL compatible hdf5 (consider cxiDB format)
• [ ] GAPDPhotonDiffractorParameters object to (de)serialize GAPD input files
• [ ] GAPDPhotonDiffractor: backengine method
• [ ] GAPDAnalyzer: Visualization of GAPD output

[ghost]

Great. Almost all the things I can think of now.

[CFGrote]

Added cmake entry. cmake -DGAPDPHOTONDIFFRACTOR=ON triggers installation of GAPD. GAPD.cuda and GAPD_mem go to bin/ the rest to share/doc/GAPD