Fixes #350 

* Replace parameter_list with mechanism_spec.
* Add prototype for mechanism parameter schema checking.
* Allow mechanism weights to be set after construction.
* Combine range parameters on density mechanisms by linear contribution in CVs.
* Cable segment electrical parameters are now member variables.
* Publish mechanism parameter information through new method `mechanism::field_info`; note this will be replaced/improved in upcoming dynamic mechanism catalog work.
* Access mechanism parameter scalars and range data via `mechanism::field_view_ptr` and `mechanism::field_value_ptr` methods.
* Allow mechanism 'global' parameters to be set via a method of specializing mechanisms (and giving them corresponding aliases).
* Extend recipe interface to allow querying of per-cell-kind global information for use by cell group implementations.
* Add unit tests for above - note that linear density mechanism parameter test is tightly coupled with the FVM discretization scheme.