pheTransitionDates function initial commit

[kjones13]

@sokole first draft of the new phenology function, ready for initial reveiw

[sokole]

@kjones13

• The convention we are using for function names is verbNoun. Like stackPlantPresence. Maybe calcPheTransitionDates?
• The convention we've adopted for this package is for functions to take list objects from neonUtilities as inputs. For example, in Dave's function, the argument is divDataList, because it gets sent the list object for plant diversity that is returned by neonUtilities. Here, maybe we could make the argument pheDataList and the function is expect the phe list object returned by neonUtilities. Then, rather than sending it pheDat$phe_statusintensity, you could just do pheDataList = pheDat in your example.
• You need to put the package prefix in front of functions used in your package that are not from base R. For example dplyr::arrange() instead of just arrange. If you run devtools::checks(), it will tell you all the instances of functions where we need to do this.

[kjones13]

@sokole in the last two commits I addressed all points above, changed name, added namespace, added functionality for using either data list or single table. Ready for re-review?

[sokole]

Sounds great. I'll take a look. Thanks!