ReducedSpaceNNFormulation: add checking on layer type

[jezsadler]

The ReducedSpaceNNFormulation class has a TODO note to check layer types when building formulation.

I need to know what layer types we support in order to implement this.

[jezsadler]

Just Dense layers with continuous formulations, not Big-M or partition.

[jezsadler]

Added check for Dense layers in my branch, will close on integration.