tasmidpoint as standin for tas

The well established indices were developed within the observational data communities. And in that context daily mean temperature, as in tas: cell_method = "time: mean", brings about a lot of complications (given the limited and shifting observation times). Hence they settled for the draconian solution that daily mean temperature is defined as (TN+TX)/2, which in terms of CF would be tas: cell_method = "time: midpoint (assuming that the time bound specify daily data!), where tas should have another name, such as tasmidpoint.

We should have the possibility to compute tas-based indices on tasmidpoint, which then should be calculated on the fly as if input data had been specified as low_data: tasmin, high_data: tasmax