`epi_cor` `shift_by` should use all non-`time_value` keys by default

Currently `epi_cor` uses only the `geo_val` field by default for computing time shifts over. If an `epi_df` has any additional keys set, they should also be used by default in the `shift_by` arg to make sure time shifts are sensical. The `shift_by` default should have the meaning `setdiff(, "time_value")`.

Start by locating the `epi_cor` entry point and inspect how its default `shift_by` value is derived from `geo_val`. Check the behavior with an `epi_df` containing additional keys; done means the default uses every key except `time_value`, while an explicitly supplied `shift_by` remains respected.

Written by the indexing model from the issue text.