r_lmms.py: decompress mmpz files

[B0ney]

Pretty much what the title says.

It gets tedious having to manually decompress mmpz files into mmp when converting my lmms projects, so I made a small modification.

With this pr, it's now possible to do something like this:

python3 DawVert.py -i test.mmpz -ot midi -o test.mid 

without the dreaded:

[error] could not identify the input format