[tanghaibao] Add support for CrossRate in models

[tanghaibao]

@MaxHalford
Thanks for the awesome package - I'm adding support to specify crossover rate. Naturally, the MutRate controls how fast one explore new solutions; CrossRate controls how much mixing amongst good solutions.

The old behavior was essentially CrossRate = 1, which, for most applications is too much. One could see this pathological behavior in the example of grid_tsp.go, where it seems to lack the ability to mutate (since crossover is dominating).

This PR allows one to specify CrossRate inside the model.

Haibao

[MaxHalford]

Please correct your code so that the Travis build doesn't fail :)

[coveralls]

Coverage decreased (-0.4%) to 98.775% when pulling 3120a02 on tanghaibao:master into cd089b9 on MaxHalford:master.

[coveralls]

Coverage increased (+0.007%) to 99.207% when pulling 47df520 on tanghaibao:master into cd089b9 on MaxHalford:master.

[tanghaibao]

@MaxHalford
While all the tests now pass, I should add that now the default CrossRate (when you don't specify it) is 0. There is a potential risk to break someone else's existing code - when they just initialize the struct without CrossRate. However, this is the same expected as MutRate, when nothing is specified, no mutations. Here when nothing is specified, no crossovers.

Or perhaps you could come up with a cleaner way to deal with defaults here?

Haibao

[MaxHalford]

Okay I'm going to merge this PR. As for the value of the crossover rate that has to be comprised between 0 and 1, that's why I impose each model has a Validate() bool method to ensure the parameters are correct. Go doesn't supports default values!