add function for taking set of back trajectory runs and determining downwind plume height and thickness estimate

[amcz]

Input should be set of back trajectory runs from the observations points (pandas dataframe with this information).

Output should be observations points with new plume height and thickness estimates.

One thing that needs to be determined is how to specify the thickness estimate. For instance it may not be sufficient to specify a bottom and top. Instead one of the columns may need to correspond to a list of heights.