Move cell description types to public includes. (#508)

Further work to public install target.

* Move SIMD classes, cell description classes, simple sampler to public include.
* Rename `cell` to `mc_cell`, `segment` to `mc_segment`, and remove `_description` from cell description class names and includes.
* Move `compartment_model` out of `mc_cell` interface and use only in `fvm_layout.cpp`.
* (Provisionally) remove area/volume methods on `mc_cell` and `mc_segment`.