NoCrossover hardcodes n_parents and n_offspring as 1, but MixedVariableMating requires it to be 2

[oxinabox]

It is fairly common to do an genetic algorithm with no cross-over.
Especially if some of your variables have weird and unusual types that are not easy to describe how to combine.
For this reason I assume the NoCrossover class was provided.

However in MixedVariableMating it requires that the crossover have 2 children and 2 parents:

pymoo/pymoo/core/mixed.py

Lines 61 to 62 in 041cc87

	XOVER_N_PARENTS = 2
	XOVER_N_OFFSPRINGS = 2

pymoo/pymoo/core/mixed.py

Line 95 in 041cc87

assert crossover.n_parents == XOVER_N_PARENTS and crossover.n_offsprings == XOVER_N_OFFSPRINGS

And NoCrossover specified that it has exactly 1 of each

pymoo/pymoo/operators/crossover/nox.py

Line 8 in 041cc87

super().__init__(1, 1, 0.0)

This means they can't be used together.