and SCEXP to SCALEINT

todo:
- gpu should only be run by master
- add documentation

disabled arrayfire from travis (but I kept the code if at some point we can make it numerically more stable)

this might decrease measured coverage, but will make the job
faster and possibly capable to finish on travis-ci

but results are very different on cpu or opencl backends

in particular a gpu version of SAXS is added in ISDB
this is just a SAXS collective variable implemented to run on GPU
a regtest is added. This was implemented with an old version of
arrayfire so it should be checked wheter it can be made more modern
a regtest is also added