Fixed typo in title
Added text to vignette
removed ggplot call used in testing
switched exp model to A*exp(-1*k)
Changed documentation:

Noted that generate_exp_guess works only with xvar-="time" and
year="relative.ion.count", even though safe_NLS will accept other
column names
*should* work with arbitrarily-named x and yvar
Creates pdf rather than html vignette

SHould change this back to rmarkdown::html_vignette to build package
properly.
removed browser lines
send xvar and yvar to generate_exp_guess
safe_NLS quit working a commit or 2 back

Not quite sure where or why, but it has something to do with sending
xvar and yvar to generate_exp_guess
Removed browsers
added subset.metab param, but did't yet implement
misc vignette changes
Added INCOMPLETE TCA character vector to data