Fastsum mex

[jenskeiner]

• added mex interface for fastsum
• split init and precomputation in fastsum for source and target nodes
• added iNFFT_1d based on fastsummex
• added mex inteface for nfsoft
• added choice of ovesampling factor no nfsoft
• small fix for doxygen

[jenskeiner]

This one is to re-merge #39 without squashing.