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
xplor nih ensemble potential calling conventions changed 2.31->2.35
get_target_atom_ids no longer updates the non bonded list

target atoms can now be accessed from the ATOM component list
remove the need to use xPyPot to access the simulation field

this requires the following patch to python/pyPot.hh, this works with
xplor 2.35.0

41,42c41
<
<   Simulation * simulation() {return sim;};
---
>
set cython wraparound=True as some arrays accesses use negative indices
in test change weights reduce requyired accuracy to 5 decimal places

changing architecture throws up this case where we system dependant  on
what the last decimal place came up with. 5dp is still a good margin of
error.
remove a print method that isn't required
add some instantiations required to build under 2.35.0 and osx

it is not currently clear which of these changes cause this need!
non bonded list now only calculates interactions for target atoms

previously it was calculating for all atoms even if they didn't have a
chemical shift restraint. This meant there were over runs on the array
bounds when a subset of atoms had chemical shift restraints
change build files to build with osx and clang 

the last set of build files suitable for linux are in commit
267f382901a3222d6495ab36707b75feecea5679 in the newar future I will
amalagamate the adhoc build systems for linux and osx
su currentSimulation for simulation as PyPot simulation is inacessible
correction, code was using weights for inactive chemical shifts