Get date_time from file structure if METADATA is missing and emit a warning

[budhaChowdhury]

This is useful to avoid the following error upon calling load_ethoscope

Error in `[.data.table`(roi_dt, , `:=`(t, (t + ms_after_ref)/1000)) : 
  RHS of assignment to existing column 't' is zero length but not NULL. If you intend to delete the column use NULL. Otherwise, the RHS must have length > 0; e.g., NA_integer_. If you are trying to change the column type to be an empty list column then, as with all column type changes, provide a full length RHS vector such as vector('list',nrow(DT)); i.e., 'plonk' in the new column.

The error occurs because ms_after_ref is a vector of length 0 because it is derived from experiment_info$date_time, which is NULL or at least not Truthy (length 0, etc) when the METADATA table is empty. The fix is to get the date_time of the dbfile from the filename or the folder itself. However, all dbfiles should have a METADATA for documentation purposes.