Allow for non-consecutive forager indices

[dimkab]

On forager object creation through the call to dataObject.__init__(), the foragersDF.forager.unique() array is implicitly assumed to be equal to range(len(foragersDF.forager.unique()). This behaviour is

1. not documented
2. undesirable in many practical situations when foragersDF is by itself a slice of a larger dataframe, in which case only a handful of foragers are present.
3. Is not explicitly checked at object creation, which causes great many issues afterwards.

The ideal solution should be to allow arbitrary non-consecutive indices at object creation and either reindex them right away, or keep track of internal indices vs. actual labels. In any case, any such assumption should be documented and asserted in code, together with a test case.

[tommybotch]

Could you provide example data of where you're experiencing this error? From looking through the code, it seems like this may be a downstream issue from dataObj.__init__() such as when it's being used to simulate trajectories.

One potential fix is allowing the user to provide foragerIDs for each forager in the dataObject (which I think is what you're saying is the ideal solution)

[dimkab]

FIxed in #168