`persistence` for resampled inits

[aaronspring]

Describe the bug
persistence gets confused on resampled inits

Code Sample

from climpred.bootstrap import _resample
he._datasets['initialized'] = _resample(he.get_initialized(), resample_dim)
skill = he.verify(metric=metric, comparison=comparison, dim=['init'], reference=['persistence'], alignment=alignment)

How to fix
in persistence in reference.py

#lverif = verif.sel(time=verif_dates[lead]) # before
lverif = verif.where(verif.time.isin(verif_dates[lead]), drop=True) # problem is that this is only a dropout solution and doesnt include multiples

Expected behavior
persistence should be possible