Issue #2 - Flux bound integration

- added methods to fbc::FBAProblem class for editing flux bounds:
 - getLowerFluxBound
 - getUpperFluxBound
 - setFluxBound
 - unsetFluxBound
 - unsetLowerFluxBound
 - unsetUpperFluxBound
- flux bound editing API made available to the Python wrappers
- added corresponding tests:
 - TestSetFluxBound.cxx
 - TestUnsetFluxBound.cxx