Magic-simplified ER microphysics modeling

[FaroutYLq]

Not sure this is the right place to do this PR. Anyway this PR provides alternative way to simulate ER microphysics, for which the physics behind is beyond the scope of this repo (notes here). Compared to traditional ER simulation, with less parameter it is way easier to converge MCMC, maybe at the cost of lower generality which is arguably small.

[coveralls]

Pull Request Test Coverage Report for Build 8587322966

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 34 of 51 (66.67%) changed or added relevant lines in 3 files are covered.
• 15 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.6%) to 84.646%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
appletree/components/er.py	4	11	36.36%
appletree/plugins/er_sr_microphys.py	28	38	73.68%

Files with Coverage Reduction	New Missed Lines	%
appletree/plugins/er_microphys.py	4	94.12%
appletree/context.py	11	91.3%

Totals
Change from base Build 8317691451:	-0.6%
Covered Lines:	2321
Relevant Lines:	2742

---

💛 - Coveralls

[FaroutYLq]

Will put this back once the physics is presented