Don't raise error if model has already been fit

[RitwikGupta]

Related to issue #3

[maciejkula]

Can I ask why you would like to have this ability?

In general, it is not clear what multiple calls to fit on the same model instance would accomplish that would not be better served by creating a new instance of the model.

In this particular case, if I didn't raise an exception the result would be building an additional set of self.no_trees trees, which is surely not what the user intends, and inconsistent with the no_trees parameter. Much better to raise an exception and prevent the user from shooting herself or himself in the foot.

[RitwikGupta]

I think this is something more warning-worthy than error causing. It makes more sense to me to print a warning and return out of the method (therefore not building an additional set of trees, I didn't include that in the PR) so the user knows what's going on, but their program doesn't suffer in any way. There are a lot of instances where you'd be working with a lot of data, costing a lot of computation time, and then you go and fit a model. If you fit it twice, accidentally, I don't think that should exit your program, losing all that time you spent re-arranging and cleaning data.

[maciejkula]

@RitwikGupta this may accomplish what you're looking for: #5

Also, speaking from experience, it always helps to run your data analysis code on a small sample first. That way you discover all the little bugs without wasting a lot of time.