Issue #4 - Flux format refactoring

- added API for getting the dictionary of exchange fluxes (i.e. fluxes containing at least a boundary condition as reactant or product) of a solved problem
- created several filters for non-boundary condition species (fbc::NonBoundaryConditionFilter class), for reaction elements (fbc::ReactionFilter class), and for exchange reaction elements (fbc::ExchangeReactionFilter class)
- removed colIndices attribute from fbc::FBAProblem class; using get_nameindex lp_solve function instead
- removed LP file loader and test
- updated CMake build tree accordingly