Add "bimonth" and "halfyear" options to round_date and friends

[jonboiser]

For #268. Did a little refactoring to re-use the formula used in rounding to "quarter".

Also, deleted that parse_unit_spec function since it really was dead code, and changed an errant tab to spaces.

Still need to write specs.

[vspinu]

Thanks. Could you please add tests for this new feature? Also the current checks fail.

[codecov-io]

Current coverage is 77.73%

Merging #414 into master will increase coverage by +0.44%

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+ Hits           1830       1836     +6   
+ Misses          538        526    -12   
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[jonboiser]

Just added tests for the new options, squashed the commits, and made sure CI passed. Let me know this needs anything else before it's ready.

[vspinu]

Why is "bimonth" quoted here? half-year should probably be "halfyear".

[jonboiser]

I quoted "bimonth", because I don't think there is a proper term for "two-month period" and "bimonth" is just a made-up word. Same with "halfyear". It might be better though if the terms were consistent with the options.

[vspinu]

Very nice!

Does this work with Date objects as expected? I know we don't many tests for that but could you add a couple of checks for Date objects as well? You can produce dates directly with ymd or make_date now.

Please also add a line to news.md with a link to #268.

This completely fixes #268, right? If so, please add [Fix #268] to the commit header to automatically close it.

Thanks a lot!

[jonboiser]

All done! I added the extra tests to the "floor/ceiling_date works for a variety of formats" to test that they work on Date class.

[jonboiser]

Had to do a couple more pushes to get the commit message right and rebuild the docs!

[vspinu]

Perfect. Thanks!