Flag plotting and manipulation

[philbull]

This PR imports some of Shaunak's flagging work into hera_stats. It has been languishing in a PR against hera_pspec for quite a while, but is probably a better fit here. Features added include:

• Function to plot multi-file waterfalls of flags and nsamples arrays
• Function to stack arrays taken from multiple UVData objects together
• Function to flag entire ranges of frequencies

[coveralls]

Pull Request Test Coverage Report for Build 134

• 281 of 300 (93.67%) changed or added relevant lines in 6 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.3%) to 93.79%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
hera_stats/tests/test_utils.py	27	28	96.43%
hera_stats/utils.py	12	13	92.31%
hera_stats/tests/test_plots.py	36	38	94.74%
hera_stats/flag.py	78	81	96.3%
hera_stats/plots.py	53	65	81.54%

Totals
Change from base Build 108:	-0.3%
Covered Lines:	1450
Relevant Lines:	1546

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💛 - Coveralls