read_MAVEN merges sample key in fxn

read_MAVEN returns a list with the dataset (with merged experimental
variables) and a vector of experimental variable names (for use later
in plotting & analysis)
Merge branch 'merge_key_in_read'
Added warnings to calc_12C

Must check that the science is correct here
Merge branch 'merge_key_in_read'
Sample key + working but brittle plot time course
meaningless change re: dropbox problem