add a class expression the xplor chemical shift reader
add a check that atoms for chemical shift restraints exist

this checks that shifts are for active target atoms, eg it will ignore
requests to calculate shifts for n and c terminal residues, and warn the
user
update to build with cython 0.20.1 xplor 2.35 and python 2.7

note this removes an unexplained crash in xplor nih ivm code which
occurs under 2.31.0 when running test_data/alvin/2L1A_csPot_new.py
add missing variable ${xc_python_version} to build_extension.sh
setup.py wasn't using xc_platform variable properly
add missing instantiation of CDSStringStreamBuf<char> to instantiate.hh

this should have been part of #e57447b025baa15e690f225d7e4ea7d836ac8401
update Simulation_simulationByID is now in simulation.py in xplor nih
update xplor nih Pot violations now returns a float_type!
make shift_io a module