added : LMTD heat exchange function, LMTDfuelHeater componenent and u…

[helenev-met]

…nit test

Goal

Describe the big picture of your changes here (one sentence).
If it fixes a bug or resolves a feature request, be sure to link to that issue (click here to know how to do that ?).

Type of change

• Bugfix (non-breaking change which fixes a bug/issue)
• New feature (non-breaking change which adds functionality)
• Refactoring (non-breaking change which reorganizes code)
• Release & Version Update (when cumulative changes justify a release)
• Documentation Update

Checklist

You can also fill these out after creating the PR, but make sure to check them all before submitting your PR for review.

GitHub checks:

• I have added the appropriate tags, reviewers, projects and linked issues to this PR
• I have checked for conflicts with target branch, and merged/rebased in consequence

Model development checks:

• I have performed a self-review of my own code
• I have checked that my changes do not bring any retrocompatibility issue comparing to MML3-DTG-v3 version
• Existing tests pass (including Metroscopia !).
• I have added/updated tests that prove my development works and does not break anything.

Library checks:

• I have checked in the file explorer that I am not adding duplicate file/folder/package (due to renaming or moving file/package)
• I have checked that the packages I upload are stored as separate files

Documentation checks:

• I have made corresponding changes or additions to the documentation
• I have added corresponding entries to the Changelog

[pierre-eliep-met]

I'm sorry I added many comments that I could have added before, I'm still learning this process, sorry for the additionnal work

[pierre-eliep-met]

A few comments again 😇
Almost there 😇

[pierre-eliep-met]

LGTM !