Make mgd77list always successful

Currently, mgd77list returns with an error when the user requests a field that is not in the data. For example:

mgd77list [ERROR]: Requested column az not in data set!

mgd77list [WARNING]: One or more requested columns not present in cruise 9701007062.mgd77 - skipping

What is skipped here is not the field, but the entire cruise.

When mgd77list is to be used in a script or another program, this behavior requires a lot of work to check what it is possible to request, and/or to get the right field in the right column. It would be much easier if mgd77list always returns a table with as many columns as requested, including NaN columns for missing fields.

When getting the result back through a VirtualFile in the API, the table is actually written in the text field of each line. It would also be more comfortable to have all the numerical fields in the data, and only the non numerical ones in the text field.

I am ready to help implement, test and maintain this feature.

Olivier