Added solver for T-s value pairs

- Props and State classes can now be instantiated with T-s pairs
Changed tolerance value for ancillary saturation check in density_Ts

- test_subcrit_twophase_consistency() works for (T,s) pairs for all
  fluids now
Added (T,s) value pairs to consistency test
Added (T,s) pair as possible input values to API doc
Use BoundedSecant solver in density_Ts

- constrain density to positive values
Merge pull request #128 from spinnau/Ts_solver

Added T-s value pair for definition of fluid states