Support for single points: a set of Omon & SImon grid_1point variables

[treerink]

Listed under #500 (comment) point 3 (Other):

ERROR:ece2cmor3.nemo2cmor: The grid opa_grid_1point consists of a single point which is not supported, dismissing variables bigthetaoga in Omon,masso in Omon,soga in Omon,sosga in Omon,thetaoga in Omon,tosga in Omon,volo in Omon,zostoga in Omon
ERROR:ece2cmor3.nemo2cmor: The grid lim_grid_1point consists of a single point which is not supported, dismissing variables siarean in SImon,siareas in SImon,siextentn in SImon,siextents in SImon,sivoln in SImon,sivols in SImon

In #500 (comment) the frequet use of them is mentioned by @zklaus.

[zklaus]

Generally that means that they just have no spatial dimensions, usually the only dimension is time. For an example, see (Omon, thetaoga) in the data request and in the tables.