Enable imputation keyvals over time units

[dwootton]

Seems like imputation over time units is a somewhat frequently requested feature. Would there by any issue with allowing the impute transform's keyVals to take arguments for generating either a numeric sequence or a temporal sequence?

As it currently is set up, impute's keyVals can only by numbers that are then passed into a sequence expression. If the key field is a temporal field, we could instead change it to compile to a timeSequence that could compile to something like this:
Vega spec of temporal imputation

Could be particularly useful when the domain can't be easily transformed into an ordinal domain (ie looking at month time unit over a multi-year period) while wanting to preserve temporal scaling.