updates to naive map code

[lea-hagen]

I came across some work I did back in June, so finally here's a PR!

This has edits to make_naive_maps.py:

• rearrange code so that it can be called with argparse or as a function
• there were a bunch of functions copied out of beast.tools.create_background_density_map, so now it just imports those functions directly

[coveralls]

Pull Request Test Coverage Report for Build 58

• 3 of 27 (11.11%) changed or added relevant lines in 1 file are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+3.5%) to 19.942%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
megabeast/make_naive_maps.py	3	27	11.11%
<!--	Total:	3	27	11.11%	-->

Files with Coverage Reduction	New Missed Lines	%
megabeast/make_naive_maps.py	1	16.18%
<!--	Total:	1		-->

Totals
Change from base Build 56:	3.5%
Covered Lines:	69
Relevant Lines:	346

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💛 - Coveralls

[lea-hagen]

@karllark this is ready to merge :)