[SUPPORT] pipeline mdh, datasources mdh and saving filtered localisations #1496

I recent looked into metadata generated when recipe modules are executed. The metadata gets nicley autogenerated from the various modules (via code triggered in the module's execute method).

Now when I save the current output of the pipeline from PYMEVisualise (File>Save filtered localizations) to .hdf the contents of pipeline.mdh gets saved with it (it seems to me). The issue to me is that pipeline.mdh only contains metadata created at pipeline generation and not the metadata of the currently selected datasource.

Several questions arise:

1. Should pipeline.mdh stay as is from generation and not reflect the currently selected datasource?
2. Shouldn't the saving with File>Save filtered localizations not include the metadata from the currently selected datasource (and possibly from Output Filter metadata as well)?

In general, what is a reasonable intended relationship between pipeline.mdh and the mdh's from the datasources in the current pipeline in PYMEVisualise?

There are also GUI questions if pipeline.mdh were mutable as the Metadata tab would possibly go out of sync, so may need some careful thinking what reasonable behaviour would be.

As is, the datasource's metadata stays essentially hidden in PYMEVisualise unless custom plugins/shell interaction explicitly were to access it.

Any ideas appreciated.