fix BDouble init?(_ nStr: String) when String has scientific notation

[piggyse]

Hi!
I think there is bug when init BDouble when String has scientific notation
So i fixd some logic and added tests
My changes passed all the Unit tests

Bug Example

       // MARK: Fail
        let a = BDouble("+2.46e+3")
        let b = BDouble(2460)
        XCTAssertEqual(a, b)
        XCTAssertEqual(a?.decimalDescription, b.decimalDescription)

        let c = BDouble("-2.46e+1")
        let d = BDouble(-24.6)
        XCTAssertEqual(c, d)

        XCTAssertEqual(c?.decimalDescription, d.decimalDescription)

        // MARK: Success
        let e = BDouble("-2.46e-2")
        let f = BDouble(-0.0246)
        XCTAssertEqual(e, f)
        XCTAssertEqual(e?.decimalDescription, f.decimalDescription)

[mkrd]

@piggy-park Thanks for your contribution :)