add function for creating back trajectories

[amcz]

Add a function (and helper functions as needed) which does the following

Input - xarray dataset representing volcat data.
output - csv file which can be used by ash_main.py to create a set of back trajectory runs from the observation points.
For each observed latitude, longitude point, trajectories at different heights should be generated.
There may be several different methods for determining what set of heights should be run at.
and the function should be written so that this can easily be changed.

[bavands]

Hi Alice,

The functions have been added to the "utilvolc/make_dasta_insertion.py" file in the branch of utilhysplit that I have forked in my repository. I had to test them to ensure that they produce the right outputs. There may be a need a few more explanations about the functions, which I would add later.

In case you want to test them using a jupyter notebook file named "plume_thickness_function_" at

/home/bavands/work-home/volcano_ash/backyard.