calculateRiparianVegetation: Calculates visible riparian vegetation metrics.
In jasonelaw/nrsa: Calculate metrics for NRSA protocol data.

calculateRiparianVegetation is used to calculate riparian vegetation metrics from the visual riparian estimates section of the NRSA forms.

1	calculateRiparianVegetation(uid, transect, transdir, parameter, result)

`uid`	a vector of site-visit indicators
`transect`	a vector of transect indicators
`transdir`	a vector of transect direction indicators which identify the bank on which the visual riparian plot was located
`parameter`	a vector of parameter names (the left column on the form)
`result`	a vector of visual riparian estimates; 0-4 for the cover estimates and (D, C, E, M, N) for the vegetation types.