Create benchmarks for swiglpk and decide on a C interface

[Midnighter]

Done: There is a comprehensive set of benchmarks (using e.g. pytest-benchmark that can be used to decide between C bindings.

Binding options in order of feasibility:

• SWIG (current)
• CFFI (Lasse has a running version)
• ctypes
• Cython
• Boost Python

N.B.: A "bottleneck" with the current interface is to retrieve the flux values for all reactions from the solver. This could also be in part to using an OrderedDict in optlang.