resolved merge conflict and made krylov sampling faster by caching matrix multiplications, thus only one matrix multiplication per iteration is needed

changed initial value for krylov sampling CG and changed demo to actually use the krylov sampling method supplied by NIFTy

Try different sampling

See merge request ift/NIFTy!245

Readability operator adapter + linear operator

See merge request ift/NIFTy!244

'mode' was used for field operator modes and operator transformation types
concurrently, which makes unclear what type of input the function operates
on. To resolve this, operator transformation type variables were renamed
to 'op_transform' and _trafo, making the distinction easy and obvious.

Poisson energy

See merge request ift/NIFTy!242

Add docker testing framework

See merge request ift/NIFTy!241