quick_mdff multiple map support

[ryanmcgreevy]

Allow quick_mdff to work on multiple input maps and gridpdb atom selections.

[ryanmcgreevy]

@trudack multiple map support is now in, and a new -gridpdbsel option is available to change from the default selection text. Both the -density and -gridpdbseloptions will take either 1 argument, or a list. You can still omit the-gridpdbseloption to use the default selections, even if using multiple maps. If you choose to use the-gridpdbsel` option, however, then you must have one in the list for each input map.