closes #345410: two_lines_rset, two_cols_rset, two_etypes_rset are badly named
replaced by multi_lines_rset, multi_columns_rset, multi_etypes_rset,
all taking optional 'nb' argument allowing to specify and exact count.
Bw compat is kept, usage of deprecated function updated in cw itself.
Also drop custom at_least_two_columns selector (in views.plots) since
it was the same as multi_columns_rset. Hopefuly the renaming will help
in avoid such pb...
We should now decide if we want to deprecate one_* selector in favor
of multi_*(nb=1). See note on this at then end of selectors.py