Improve handling of time

[dblodgett-usgs]

Probably for discussion, but the time handling could be better. I've spent a lot of time with reading and converting time in netcdf over here: https://github.com/jjvanderwal/climates/ Search the repo for 'chron' and 'julian' ugh, so annoying.

I like the current 'just pass in a nice POSIX vector', but I think we could take it a step further and give optional control over the date origin / step with some of the smarts I developed for that climates package.

[dblodgett-usgs]

Start by bringing over these functions:
https://github.com/jjvanderwal/climates/blob/master/R/get_time_dim.R
https://github.com/jjvanderwal/climates/blob/master/R/CF_date_range.R and https://github.com/jjvanderwal/climates/blob/master/R/request_time_bounds.R