Simplify plot_missing API

[x] intelligent defaults for finding stuff like "gwcode", "date", "year" so that those arguments don't need to be specified
[x] parsing dates to identify year/month/day periods so that 'by' need not be specified
[x] change period from default "year" to default NULL with id function
[x] add partial option
[x] intelligent default for partial option: if time variable is YYYY or YYYY-MM set to "any"
[x] by default if x is NULL, use all non-ID columns

andybega / states