[Users] EOS_Omni parameter file conversion guide draft

I began to write an EOS_Omni parameter file conversion guide, mostly because I needed it myself. I am writing this guide as part of the EOS_Omni documentation. While writing this guide, I also wanted to document some of the details of EOS_Omni, and I found some unexpected behaviour. I attach my current draft; I would appreciate it if someone with more experience than me could have a look at it, correct the larger errors, and then commit it to the thorn.

I am particularly confused about the meaning of the *_ini parameters (nothing in this thorn makes a distinction between initial data and time evolution), and the meaning of have_temp (which seems to ignore the temperature either way).

-erik