Unresolved Overloaded Function

[akleeman]

When using an overloaded function with the group_by methods the compiler isn't happy. We get a long string of errors which are relatively easy to identify since this message repeats:

albatross::RegressionDataset<Feature>::group_by(<unresolved overloaded function type>)

but it'll be buried in a long string of template candidate failures which makes it tough to spot.

Ideally we would:

• Support overloaded functions (not sure if that's possible)
• Give a more informative error message. (this could contain a suggestion to wrap the desired function in a lambda with the specific type desired).