Replace em_morphology middle layer API. (#918)

* Consolidate embedding-dependent morphology data in new class `embed_pwlin`, which provides sample location lookup, handles branch and radius queries, and performs piecewise-constant integration over a branch by length, branch surface area, or inverse cross-sectional area.
* Extend region and locset expressions to represent named regions/locsets.
* Rearrange morphology-related functionality so that morphology-independent functions are provided via `morph/primitives.hpp` and topological functionality via `morph/morphology.hpp` and `morphology` methods.
* Provide new class `mprovider` which gathers a morphology, its embedding, and a concrete label dictionary.
* Populate mprovider concrete region/locset maps at initialization time, throwing exceptions if there are unresolved references or circular dependencies.
* Gather and unify morphology-related exceptions in `morphexcept.hpp/cpp`, unifying error text and providing programmatically readable associated d...