Improve pandas.DataFrame.resample primitive

[csala]

Make several improvements to the pandas.DataFrame.resample primitive:

1. Make the aggregation argument a tunable hyperparameter with a list of allowed options.
2. Allow using custom functions as the aggregation instead of one of the predefined options.
   • If a callable is passed, apply it.
   • If one of the allowed options is passed, get the corresponding function and apply it.
   • Otherwise, try to import the indicated object and apply it.
3. Make the rule argument a tunable integer hyperparameter, which should be interpreted as a number of seconds, but keep backwards compatibility by supporting being passed a fixed string directly.