New factor block difference

Implementation of a new type of factor defining the euclidean difference between 2 stateblocks. This is a analytic factor (jacobians are trivially +/-Identity). As the factor_block_absolute.h, on which this is inspired, it offers the possibility to constraint only segments of these 2 stateblocks, if these segments are consecutely stored in the stateblock (ex: for a position stateblock, it is possible to constraint only xy or yz but not xz). This could be implemented if necessary.