Make the ScalingOperator consistent with `from_global_data`, `full` and
possibly other operators which take the domain as first argument. This
breaks the currently enforced consistency with the `DiagonalOperator`
which takes a field as first argument and the domain only as an optional
keyword argument.

Fixes #279.

This is a performance tweak. Since inverse diagonal operators are automatically
converted to a pure diagonal operator with the inverse on its diagonal, this
commit saves memory.

BROKEN FOR MPI if there are NaNs in the input

corrected some conceptual mistakes in StandardHamiltionian that had no effects because the prior metric is 1

changed Inverse Gamma likelihood to use the parameter convention from wikipedia thus making it more general. Also added documentation

Reviewed docs on operators, library, sugar. Fixed bug in makeOp due to the recent BlochdiagonalOperator interface change