Adding type class for operator map for type configurable parser

[fshao816]

Here's a PR for your consideration; basically made the operators map an implicit. Note that this does change the way you call MathExp.parse however:

MathExp.parse[Double](str)

You'll have to explicitly supply the type since there's no way to infer it.

The way we are using it with saddle looks something like this:

import org.saddle.Vec
import org.saddle.ops._

    implicit object OperatorVector extends Operator[Vec[Double]] {

      val unary: Map[String, Vec[Double] => Vec[Double]] = Map.empty

      val binary: Map[String, (Vec[Double], Vec[Double]) => Vec[Double]] = {
        Map(
          "+" -> (_ + _),
          "-" -> (_ - _),
          "*" -> (_ * _),
          "/" -> (_ / _)
        )
      }
    }

We'll be building out unaryOps (for saddle Vecs) in the future.
Nice work on the library!

[facaiy]

Cool, @fshao816 ! The PR help to handle both Vector and Double, right?

I'm glad to take a review, however, CI run failed. Please check the Details links at the bottom.

Moreover, we'll be more confident after those points below are checked:

• code style: follow Code Style Guide of Spark
• add some unit tests to verify your changes.
• add/modify documents (README.md) if necessary.

Be free to ping me if you need more help. Thanks.