Use a more explicit default for `only_rotate_len_k`

[fpietra]

Currently, in propagate of MonomialPropagator, we use k=0 to disable the effect of the option only_rotate_len_k. k=0 is also the default value. This can be confusing, and could be replaced with None to denote more clearly that the option is disabled and no filtering is done on the lengths to process. k=0 can then explicitly denote the trivial case of the operation that filters out all lengths.

EDIT by @SimoneGasperini: we also need to add validation on the value passed for this optional argument (e.g. 0 $\leq k \leq$ cutoff) and explain a bit better what it does in the docstring. Optionally, more tests can be added to check the behavior with different values of $k$.

[adamglos92]

In #121 it is kind of done, but the interface change should be transferred to C++ eventually