Clean up the bundled mechanisms!

[halfflat]

Mechanisms currently included in the default catalogoue, such as hh, have various horrible NMODL descriptions. If nothing else, they make documenting correct usage much harder than it should be.

Proposal:

• Get in there and clean up spurious GLOBALs and ASSIGN etc.
• Restrict range parameters on density mechanisms to conductances and channel densities.
• If we need to keep some for benchmarking purposes, we move them into their own catalogue.

[halfflat]

Some progress already — thanks @thorstenhater! — part of the clean up though may depend upon the resolution of #1380.