Support for repositories with multiple GEMs (f. ex. strain collections)

[famosab]

Description of the issue:

So far only singular GEM repositories fit within the standard-GEM template. While some parts can easily be used with multiple GEMs, such as the overall structure, other things fall short when multiple GEMs are hosted in one repository.

Expected feature/value/output:

There are two ways how to approach this:

1. One could either extend this repository to be applicable to repositories with multiple GEMs
2. Or create a separate template for multiple-GEM-repositories

A few changes that directly came to my mind are:

• the model folder would need to change to models
• one model format should be stored in the models folder (f. ex. xml) conversions into other formats could be done via github actions and stored either in subfolders (within the models folder) or different folders in the repository (f.ex. a folder called json for all json models etc.)
• the naming convention for the GEMs needs to be adapted
• as a possible Memote report, a memote report diff would be preferable over reports for every model

Current feature/value/output:

Not available.

I hereby confirm that I have:

• [X] Tested my code with all requirements for running the model
• [X] Done this analysis in the main branch of the repository
• [X] Checked that a similar issue does not exist already
• [X] If needed, asked first in the Gitter chat room about the issue

[draeger]

Just a very minor note: I'd keep the model directory without the plural form. Rationale: When talking about the source code of a program, we also typically refer to multiple source files. Still, it is not called source codes. The singular is appropriate here as an unmarked form.