fix: save ec model xml

[mihai-sysbio]

Main improvements in this PR:

This PR aims to resolve a bug where .xml was unnecessarily appended to a filename that already ended with .xml when saving an EC model. Also, a typo has been also fixed.

Instructions on merging this PR:

• This PR has develop as target branch, and will be resolved with a squash-merge.
• This PR has main as target branch, and will be resolved with a merge commit.

[github-actions]

This PR has been automatically tested with GH Actions. Here is the output of the tests:

 
GECKO installed, unknown version (cannot find version.txt)
Running geckoCoreFunctionTests
Done geckoCoreFunctionTests
__________

Note: In the case of multiple test runs, this post will be edited.

[ae-tafur]

Looks good.