If pmp2sdp --input is a directory, process all files inside it

[vasdommes]

If there are several input files for pmp2sdp, currently one has to create .nsv file with the file list.
It would be more convenient to and pass the directory path as --input, if all files are in the same directory (which is usually the case).

[vasdommes]

NB: for stable output, we should process files in a lexicographical order.

[vasdommes]

Also we should ignore .nsv in this case