refactor plotters and writers to use check_outdir() consistently

[Thomas Gilgenast]

currently, tools are expected to call check_outdir() individually before attempting to write any files

one alternative would be to consistently make all writers and plotters call check_outdir()

among other things, this would remove the currently-awkward calls in some tools that call parallelized versions of plotters and writers where a for loop must iterate over some dict that specifies the outfile for each region

this is pretty low priority because things are more or less working with the current setup since each tool can manually call check_outdir() instead

[Thomas Gilgenast]

cross referencing #11

[Thomas Gilgenast]

major overhaul of counts/primermap parsing and plotting API

plotting API is now formalized in a new decorator: @lib5c.util.plotting.plotter

most plotters now follow this API, except for the old heatmap plotters and cluster heatmap plotters, as well as legacy plotters

primermap/counts parsing is now standardized in line with modules2016: only two functions are supported: load_primermap() and load_counts()

all tools have been updated to reflect this change

lib5c.tools.helpers.resolve_level() has been broken up into lib5c.tools.helpers.resolve_level() and lib5c.tools.helpers.resolve_primerfile() - this helper is only needed in a small number of tools

lib5c.tools.parents.level_parser has been broken up into lib5c.tools.parents.level_parser and lib5c.tools.parents.primerfile_parser - level_parser is used rarely while primerfile_parser is ubiquitous

this closes #23, closes #21, closes #18, closes #22, closes #11, and closes #9

→ <<cset f4e899afa042>>