Mesoscopic surface regions

[adamjgnoel]

Current implementation of surface regions is in the microscopic regime only. In fact, microscopic surfaces cannot even be neighbors of mesoscopic regions. Mesoscopic surfaces will require some special consideration, namely:

• definition of "excluded regions" (would apply to micro or meso). These would create "holes" in existing regions
• potentially a separate definition scheme for chemical reactions in mesoscopic surfaces, in terms of how/whether molecules attach or are released. This may be achievable just via adding #22.

[adamjgnoel]

Deferring milestone for this issue. Updating reaction propensities for surface reactions is non-trivial. Should refer to how this is done (if at all) in existing mesoscopic tools. Otherwise, may need to derive directly.