Add cached Sun/Moon calculations in `Observer` class #156

Job	Run time
Code style checks	19s
Python 3.7 with minimal dependencies	1m 12s
OS X - Python 3.9 with minimal dependencies	1m 18s
Windows - Python 3.11 with all optional dependencies	2m 17s
Python 3.11 with remote data, all dependencies, and coverage	1m 20s
Python 3.11 with latest dev versions of key dependencies	2m 15s
Test building of Sphinx docs	2m 5s
	10m 46s

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