Replace `fill_through_version`, `truncate_versions_after` with a generalized `fill`/`complete`

[brookslogan]

Interacts with / supersedes #272, #231. Hope to enable to do filter/completion on epikeys, time_values, versions, version-lags via different parameters. Note that the version-related things are not as simple as others.

[brookslogan]

When is this used? As part of epix_merge on non-equally-up-to-date archives, and maybe when performing production forecasting and wanting to epix_as_of a particular date that may or may not have version data reported by the forecast generation time.

We might not want to generalize these functions; filling in time_values might be more of a modeling decision that should be made elsewhere.

[dsweber2]

I've implemented a "fill at the end" in cmu-delphi/epipredict#296 for time_value locf, which we could probably adapt to something here. I just found myself reusing a large chunk of that code to convert weekly data to daily data, so it may be useful more generally