SPEC: Lcmgen changes

[leshy]

very interested in a solution to subclassing and optional convinience methods.

In terms of code gen, if choosing to avoid awkward manual cpp string manipulation:

• A lot of this might be achievable programatically by a base metaclass or class decorator,
• Alternatively can use python itself to modify the cpp generated code using AST module or something

[leshy]

implemented, closing #417