Remove variable neighbors (factors) by default on deleteVariable

[Affie]

If a variable is deleted from DFG object, then default is to delete surrounding factors also. This can
be avoided with a keyword argument.

If a factor is not deleted and a new variable is introduced, then a new factor must be created by one of the verbs and replace the current factor which has (by this time) a stale but immutable _variableOrderList.

One reason is that deleting and adding variables will not preserve the all important order. Another reason is that deleting both the edges in the graph and variables from this list undermines the test where one compares the two lists against each other -- currently used for orphan test. Third reason is when using the multhypo interface, then the number of variables is specific to that instance of the factor (as in all cases really) and therefore _variableOrderList must be immutable.

Originally posted by @dehann in #20 (comment)