Refactor how type conversion works

[klauer]

TypeConverter class, TypeConverterId can be awkward and unnecessary boilerplate.

Some preliminary discussion in #43 (comment):

I was thinking that class inheritance information could be used to indicate conversion compatibility, perhaps. At least in relatively simple graphs, it may even be possible to omit data_type = NodeDataType(...) then. That's aiming toward my hopeful overall goal of less boilerplate.

But data types may still be a necessity:

Perhaps, but there might be some very exotic conversions (e.g. an image into a text) and then the inheritance wouldn't help that much.