IQMs plugging pattern

[oesteban]

This discussion is open in replacement of #178 to sketch out how a plugging pattern could be implemented.

In my mind, this probably requires that we establish an API where we can create a dictionary of interim results on which metrics can be computed.

Then all 'pluggable' metrics would be within one module and we could dynamically load all functions in that module and run them passing the dictionary where variable names are standardized.

Similar introspection work is necessary to standardize the conversion of the output of these functions into something workable by the ComputeIQMs interface.