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

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