Bugfix

* added na.rm=T to the sum functions
Added the maximum sequence length criteria.

* the former version was prone to selecting the same crop again and again,
	but, as for the offset year, that does not make sense.
* we now fetch the maximum number of years from a table a single crop might be followed
 his own kind. If that number is exceeded, the probs for the same kind are reduced.
* that does not mean, that crops can not be the same for more than those maximum years,
	if there is no other crop availabel, there is not chance to avoid it.