Heavy unit tests and examples

Some of the unit tests and examples request much time to run, and it's not ideal for efficiency (and could be questioned by CRAN.) In addition, the pipeline could fail due to timeout (1hr). Take this run as an example, https://earth.bsc.es/gitlab/external/cstools/-/jobs/21834, it took 52 minutes 20 seconds to finish,

(line 168)
Examples with CPU or elapsed time > 5s
                                 user system elapsed
QuantileMapping               178.856  0.006 178.720
CST_QuantileMapping           169.801  0.053 169.725
CST_EnsClustering              61.552 11.343  60.378
CST_MultiEOF                   23.750 38.011  13.955
CST_CategoricalEnsCombination  37.573  0.023  37.568
CST_MultiMetric                15.422  0.173  16.788
EnsClustering                   7.659  0.652   7.562
PlotForecastPDF                 5.132  0.003   5.132
* checking for unstated dependencies in ‘tests’ ... OK
* checking tests ...
  Running ‘testthat.R’ [20m/20m]
 OK

It shows that unit tests take 20 minutes to finish, and some examples in the functions are too heavy.

We should check if these pieces of code can be improved.

FYI @erifarov