add a test for ticket #66: bug 'chemical shifts for 1st and last atoms
improve message for #66 bug 'chemical shifts for 1st and last atoms
bug fix order of loading RDC and xcamshift force-field (#68)

fix forcefield so the NB it can ignore atoms with an unknown chemical
type

close #68
bug #64 if the files directory containing the force-field isn't found...

now raises a clear exception rather than crashing if the force field
database isn't found 

close #64
add more search paths for the database files

1. <xcamshift-distribution-root>/data
2. <xplor-home>/databases/xcamshift
bug #65 allow xplor comments: ! in restrainst files  

close #65