Add warnings when using smoothNMF

[adriente]

Issue

It can be hard to understand :

• what each parameters of smoothNMF does
• what is the relationship between parameters

Example :
It does not make sense to put simplex_W = True and simplex_H = True.

Suggested solution

It would be great to build an object/function that inspects the parameters of smoothNMF and warn the user about possible missuses.

non-exhausistive list of possible conflicts

1. simplex_W = True, simplex_H = True
2. simplex_W = True, fixed_H is not None
3. simplex_H = True, fixed_W is not None
4. G = None, simplex_W = True
5. The strength of lambda_L, mu, epsilon_reg is much lower/higher than the Poisson loss. (to be checked after a few iterations)