Change form of assets produced in EGU Copernicus tex outputs

[stevejpurves]

Based on a recent submission using this template, the following feedback was received from the editors:

1. "Please send me an update ZIP file with the figures named according to figure number (e.g. fig1.png, figA1.png). This is important for our internal workflow. While you are at it, you can also adjust the names in the tex file."

So rather than producing hash based filenames, in this instance a structured approach to naming image assets and referencing them in figures is needed.

2. Please send me one file for your tex file. Currently, your manuscript is made of several different elements.
   We should allow for a single .tex file to be produced if it does not already.