## Counting exceedance above/below threshold across differernt datasets.

We have numerous indices counting exceedance above/below a fixed threshold. There is need to be able to carry out such calculations across different datasets that may have different reference climate (i.e. there are "biases"...). In a simplistic way, and without invoking bias adjustment of the input data, this could be done by carrying out a quantile mapping of the threshold applied to the reference data. For this we new a couple of new index functions

We need a couple of new functions:

- index function to calculate a percentile rank based on a fixed threshold, e.g.
`tx{TT}rank`

etc. This have to be done over some reference period, typically a 30 year period like 1971-2000. - index function to calculate a percentile value based on
**spatially varying**percentiles (e.g. produced by 1.). This have to be done over a reference period, which typically is the same as used in 1. - index function to count the number of exceedances (above/below) a
**spatially varying**threshold (e.g. produced by 2.)

In step 2 it should be possible to read the percentile array from the file produced by 1. Step 2 and 3 could be combined. Irrespective of this, the result of 2 should be an auxiliary coordinate in step 3.