Added meta data to the output of derived field computations and to the difference data source.

Closes #160 (closed).

A recent merge request !287 (merged), also allowed MStructedGrids to include meta data on their processing information. That merge request added computation information of the smooth filter and the derivative filter to the grid, and therefore also to the exported NetCDF file.

This MR also adds this information to the other processing sources in Met3D: The DifferenceDataSource and the computation of derived variables. The latter one now adds the list of source variables to the computation information, and in the future specific processors (e.g. computation of wind speed) can also add further processing information (e.g. the unit of the resulting grid).