[summary] Warn about sub-second timesteps

It is possible in Eclipse to take sub-second timesteps that are exported to UNSMRY-files. Sub-second accuracy seems not supported in `libecl`, and in `ecl2df` it will end up as a repeated time index at the same second. This easily gets the user into trouble, and at least a warning could be issued when this (sub-second timestep) seems to be the case.

Start by tracing how ecl2df reads UNSMRY files and constructs the time index, especially where sub-second timestamps are converted to seconds. Confirm the repeated-index behavior described in the issue, then add a warning for detected sub-second timesteps and verify that the warning appears without changing other timestep handling.

Written by the indexing model from the issue text.