Add sequence command

[trudack]

As the gap finder needs to convert pdb into a one letter code sequence this should be a separate command as well like

modelmaker sequence -model .pdb

The output file should be .seq

Max also wrote a script to write out parts of a sequence file. This could be another feature of sequence like

modelmaler sequence -sequence .seq -start 23 -end 306

Output _start-end.seq

[ryanmcgreevy]

Both of these commands have been implemented as pdb2seq and seqsub.