Additional tests SelectPeriodOnDates and SelectPeriodOnData

Hi @lpalma and @amanriqu,

The functions SelectPeriodOnDates() and SelectPeriodOnData() have been developed and there are some tests for the Decadal Predictions and Seasonal Forecast: https://earth.bsc.es/gitlab/es/csindicators/-/blob/master/tests/testthat/test-SelectPeriod.R

Could you add some tests for the Subseasonal Forecast case?

Please let me know if you find any problems.

From my side, this is not urgent but it is a very important feature of the package.

Cheers,

Núria