rsl/tsl attribute sampling in netCDF_CML

[fenclmar]

rsl/tsl variables could have attribute "sampling" or something like that, which would indicate, if the value is instanteneous or if it is some statistics describing observation intervals. Attribute would equal "Instantaneous" for instantaneous sampling and "aggregated" or something like that for min/max or any other statistics.

Details about sampling could be also stored in global attributes, e.g. in the commment. If I remember well, some older NMS were providing also rsl_avg and there might be other statistics used to describe observaiton interval. If some other statistics than min/max is needed, user can define new varible name, e.g. rsl_avg, or rsl_median and desctibe it (including sampling) in the global attribute "comment".