Removed unsafe hack in XFunctionInternal
Refactored derivative logic, #907
Fixed failing unit tests, #884
Revert "Commented out failing unit tests, #884"

This reverts commit 6138273f48bc92043dd75fa69e4548187f591cf3.
Removed unused (deprecated?) option 'sparse', #884
Removed deprecated option 'store_jacobians', #884
Removed SparsityGenerator (FX::setJacSparsity can be used instead), #909