- added cross validation option to cmake files
potential fix for #1897

initialize loop, hope this is correct. At least it does not crash now.
Fixed Bug 1897. The calculation of the validation experiments follows now
closely the calculation of the fitted experiments, i.e., intermediate steps
are calculated and all improvements and bug fixes in the fitted experiments
are propagated.
fix for #1899: sensitivities task disables "update model" for subtasks

It makes no sense for a subtask of a sensitivity to change the initial
state of a model