Test for the qstep bug appearing at large rstep.
Redo rlimits cache when input parameters change.

This fixes bug in qstep update after rstep change.
Test qstep updates when qmax is very small.
Update optimum qstep when qmax changes.
DebyePDFCalculator::setQstep need not be virtual.
Revert to optimum Qstep when set to that value.
Serialize cache validity flag.

Keep backward compatibility to support old Python pickles.