Species State Rounding Fix

[jtcooper10]

Fixes the issue where numerical errors are introduced with continuous values when being evaluated for stochastic reactions. It's fixed by making the simulation's current_state array be exclusively double, instead of using a double array for deterministic reactions and a separate int array for stochastic ones.

Changes

• All instances of current_populations vector in C++ solver are removed
  • Any areas that were previously dependent on current_populations (int vector) now use current_state instead (double vector)
• map_ode_propensity and map_ssa_propensity are added to the template file, which allows the hybrid solver to explicitly specify which propensity to use
  • In contrast, previously the decision of "which propensity to use" was based on a function overload of map_propensity, where the decision was made based on the type (double or int) of the state vector
• Integrator RHS always uses the integrator state in propensity functions, regardless of reaction type
• Added propensity methods to HybridReaction class, to reduce code duplication
• Added Tau select overload that accepts double as state vector
• Added unit test to check for rounding errors in continuous species

Fixes

• Stochastic reactions no longer truncate continuous species values, which was preventing certain reactions from firing