Add heat kernel #1900

Job	Run time
Conda Pytest Symengine	15m 16s
Typos	6s
Conda Pytest Symengine with Loopy FFT	10m 43s
Conda Pytest	16m 54s
Tests for downstream project pytential	37m 30s
Conda Examples	2m 22s
Ruff	8s
basedpyright	2m 55s
Documentation	2m 35s
	1h 28m 29s

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