Pepteditor improvements for GPCRs.

[primaryodors]

• UPRIGHT command now accepts an optional argument containing strand IDs to remain with the working strand as a unit;
• New BWCENTER command for aligning protein models based on the locations of Ballesteros-Weinstein n.50 residues;
• Allow negation of IF with keyword NOT;
• IF EXISTS and IF NOT EXISTS commands for checking for presence of files;