mrke/NicheMapR: NicheMapR v3.1.0 - Now including an official release of the endotherm model

This is the official release of the endotherm model of NicheMapR, timed with the publication of the accompanying software note in Ecography:

Kearney M. R., Briscoe N. J., Mathewson, P. D. & Porter W. P. (2019) NicheMapR - an R package for biophysical modelling: the endotherm model. Ecography 43: 1-11. doi: 10.1111/ecog.05550

It includes major corrections to the equations for conduction from the previously released version, with the vignette describing all the equations in the model updated accordingly along with some other minor corrections.

It also rectifies an error that crept in to version 3 of NicheMapR's ectotherm model, whereby the egg batch buffer in the DEB model was filling faster than it should have been.