Bump extern/Microphysics from fb8b008 to 397d3d2 (#787)

Bumps [extern/Microphysics](https://github.com/psharda/Microphysics)
from `fb8b008` to `397d3d2`.
<details>
<summary>Commits</summary>
<ul>
<li><a
href="https://github.com/psharda/Microphysics/commit/397d3d206d0de0f494a728279f448ba6485c37d5"><code>397d3d2</code></a>
Add cosmic rays to metal chemistry (<a
href="https://redirect.github.com/psharda/Microphysics/issues/1651">#1651</a>)</li>
<li><a
href="https://github.com/psharda/Microphysics/commit/ca10c3ead328230f1844f70eb7aac6bacfebd137"><code>ca10c3e</code></a>
Update README to mention ISM chemistry (<a
href="https://redirect.github.com/psharda/Microphysics/issues/1652">#1652</a>)</li>
<li><a
href="https://github.com/psharda/Microphysics/commit/01cc637f10e8b2079059514f609d244152b18eca"><code>01cc637</code></a>
Correctly handle ices in metal chemistry (<a
href="https://redirect.github.com/psharda/Microphysics/issues/1650">#1650</a>)</li>
<li><a
href="https://github.com/psharda/Microphysics/commit/1e1dc1440489abf82c55247b72507bb9bf15740d"><code>1e1dc14</code></a>
Remove unused param (<a
href="https://redirect.github.com/psharda/Microphysics/issues/1648">#1648</a>)</li>
<li><a
href="https://github.com/psharda/Microphysics/commit/d98d39bd2f0d97ac2ad0dba554b9ffc787001253"><code>d98d39b</code></a>
Define Tdust as aux var (<a
href="https://redirect.github.com/psharda/Microphysics/issues/1649">#1649</a>)</li>
<li><a
href="https://github.com/psharda/Microphysics/commit/3961b439ba5c8193975018300e4c23a09b533206"><code>3961b43</code></a>
update changes for 24.09</li>
<li>See full diff in <a
href="https://github.com/psharda/Microphysics/compare/fb8b0080c25fba6b224fc188ece89f962a34cc7f...397d3d206d0de0f494a728279f448ba6485c37d5">compare
view</a></li>
</ul>
</details>
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