The derivatives are computed with respect to transforms parameters.
This is the first step towards los calibration.

Thanks to Silvia Ríos Bergantiños and Beatriz Salazar for the translations.

Thanks to Francesco Coccoluto

Thanks to Lucian Barbulescu for the translation.

This method allows caller to make sure a direct location call will not
generate an exception dut to date out of range.

tStep is really related to minData, maxDate, so it ought to be alongside
with them.