F1, precision & recall for multi-class classification

[sandervh14]

Is your feature request related to a problem? Please describe.
I am evaluating a multi-class classification case. The values in the target column and prediction columns can be 0, 1, 2, 3 or 4.
The FScore, Precision and Recall calculation results in the following error:

java.lang.IllegalArgumentException: Precision can only be applied to binary classification: 1000

	at smile.validation.metric.Precision.of(Precision.java:54)
	at smile.validation.metric.FScore.of(FScore.java:90)
	at smile.validation.metric.FScore.score(FScore.java:47)

Describe the solution you'd like
Could you implement this, please? Calculating precision and recall for multi-class classification should be possible.

Describe alternatives you've considered
Not applicable.

Additional context
Not applicable.

[haifengl]

Sure, we will add support for multi class. BTW, smile.deep.metric package precision and recall support macro-, micro-, and weighted metrics of multi-class data.

[haifengl]

It is added to master branch now. See tests for examples.