Remove boilerplate for plugin instantiation about gmxapi

[eirrgang]

The resources a plugin receives will either have to be templated on the plugin or have a whole lot of casting and/or run-time type stuff, I think, so I'm going to first explore templating. The early result will look like I am replacing the class that the developer previously defined with a class I cobble together with templating, I expect.

• developer defines a struct to hold their state. The types it can hold are limited to types our API knows how to serialize.
• developer writes the functions they want called. State variables are instead accessed through a member (with the struct type) of a (templated) resources argument passed in.
• developer expresses a template to connect the pieces.
• developer calls functions on the resulting object for each member of the state data structure to declare their keyword name, type, behavior, and initialization.

I don't know how to check for uninitialized struct members in this case. We might be able to build up a constructor call that would only compile if we are able to use the automatic POD aggregate constructor.

It isn't worth conjecturing further until I experiment a bit.