Pygeopack

Added DOI and citation file.

Minor changes include additional functions.

Added a function to plot some field traces in the XZ plane (`PyGeopack.Test.PlotTraces()`).

Also fixed a bug with saving field traces

Bug in ModelField() should be fixed in this version.

This should hopefully fix the bug where solar wind velocity needs to be manually entered in order to convert between coordinate systems.

I recommend updating the parameters using `gp.Params.UpdateParameters()` after installing this code.