Cmake updates and python installation best practices

[bennibbelink]

This includes lots of minor cmake changes and attempts to use modern python best practices for installing cyclus' python package

The apt-20.04 build_test.yml fails because the dependency images don't have pip installed, but it passes in the build_test_publish.yml. Should I make a separate PR to add pip as a dependency, then address this PR so the workflows pass?

[bennibbelink]

#1633 will add pip as a dependency, then these workflows should pass on the new images

[coveralls]

Pull Request Test Coverage Report for Build 7061291662

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 9 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.001%) to 49.335%

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Files with Coverage Reduction	New Missed Lines	%
src/pyinfile.cc	9	48.94%

Totals
Change from base Build 7027753240:	-0.001%
Covered Lines:	23971
Relevant Lines:	41349

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💛 - Coveralls

[gonuke]

Thanks @bennibbelink