Commit a141d2bf "resolve degenerate use of 'mode' in OperatorAdapter
and LinearOperator" was pushed without ensuring consistency with
other operator definitons and correct exception strings. This commit
aims to resolve this.

The argument variable of _flip_modes(…) is now always called 'trafo'
and the Exception thrown for invalid values of it is
ValueError("invalid operator transformation"). To aid readability,
numerical constants were replaced with correspondingly named
constants (ADJ, INV).

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

'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.