Return types in Endpoint.on_get_request() should be expanded

[nsoblath]

Currently only values are allowed as return types. But we could expand that to cover anything that can be converted into a param type.

dripline-python/dripline/core/endpoint.py

Line 51 in c0e1e1c

the_node["values"].push_back(scarab.ParamValue(an_attribute))

Let's allow an_attribute to be anything convertible to param.

And perhaps if it's not convertible to param we try to convert it to a string representation. That should cover most cases.

Effectively:

try:
    converToParam(attribute)
catch [maybe RuntimeError?  or did we create a custom translator for scarab::error?]
    convertUsingRepr(attribute)