Microphysics emulator

[elynnwu]

Purpose

Describe the purpose of this PR and the motivation if relevant to understanding. Include links to related issues, bugs or features.

Remove the sections below which do not apply.

Code changes:

• Provide a list of relevant code changes and the side effects they have on other code.

Requirements changes:

• Provide a list of any changes made to requirements, e.g. changes to files requirements*.txt, constraints.txt, setup.py, pyproject.toml, pre-commit-config.yaml and a reason if not included in the Purpose section (e.g. incompatibility, updates, etc)

Infrastructure changes:

• Provide a list of changes that impact the infrastructure around running the code -- that is, changes to Makefiles, docker files, git submodules, or .jenkins (testing infrastructure changes). If Jenkins plans are also being manually changed, indicate that as well.

Checklist

Before submitting this PR, please make sure:

• [ ] You have followed the coding standards guidelines established at Code Review Checklist.
• [ ] Docstrings and type hints are added to new and updated routines, as appropriate
• [ ] All relevant documentation has been updated or added (e.g. README, CONTRIBUTING docs)
• [ ] For each public change and fix in pace-util, HISTORY has been updated
• [ ] Unit tests are added or updated for non-stencil code changes

Additionally, if this PR contains code authored by new contributors:

• [ ] The names of all the new contributors have been added to CONTRIBUTORS.md

[FlorianDeconinck]

README.md updated to point to the branch for future reference