Add cached Sun/Moon calculations in `Observer` class #155

Job	Run time
Code style checks	17s
Python 3.7 with minimal dependencies	1m 14s
OS X - Python 3.9 with minimal dependencies	1m 29s
Windows - Python 3.11 with all optional dependencies	2m 26s
Python 3.11 with remote data, all dependencies, and coverage	1m 27s
Python 3.11 with latest dev versions of key dependencies	2m 20s
Test building of Sphinx docs	2m 16s
	11m 29s

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