New exponential change fxn definition changes

Changes below are still undocumented:

Changes to the following functions to reflect new definition of exponential rate function:
branchMeanRateExponential
exponentialRate
timeIntegratedBranchRate
getEventData
subtreeBAMM
Changes to the following functions to remove environmental variables
dtRates
assignColorBreaks
colorMap
rateLegend
plot.bammdata
speciesByRatesMatrix
Changes to the following functions for faster performance
getDesc