Replace the gregorian package

[ThomasThelen]

gregorian isn't on cran an we can't submit until it is-so let's replace it with lubridate

[zizroc]

Let's just drop this. Having dates doesn't presently add value to the package and it's actually opposing generalization: it's a holdover from when we were thinking about exogenous drivers of change from the climate, and it imposes a calendar system that would have been totally alien to Maya. We can just do time steps with arbitrary labels -- days by default (because of how rates are presently entered), but could be anything. Also in case someone wants to subclass in a way we're not predicting. Why limit? In the future, we can write a few functions to reconstruct Mayan/Gregorian/Julian calendar dates from the output days.

[ThomasThelen]

Done in 014869b