Extended expression parser for the [dev.to daily challenge #10](https://dev.to/thepracticaldev/daily-challenge-10-calculator-23n7).

Try it:

```
% erl
1> c(calc).
{ok,calc}
2> calc:test().
  All 57 tests passed.
ok
3> calc:calc("1.5 * 2").
3.0
4> calc:calc("12e4 * 2e-3").
240.0
5> calc:calc("3 ^ 4").
81.0
6> calc:calc("2 ^ 0.5").
1.4142135623730951
7> calc:calc("2 + ( 3 * 4 )").
14.0
8> calc:calc("8 / 2 * (2 + 2)").
16.0 % OBVIOUSLY!
9> calc:calc("3^(1/2)").
1.7320508075688772
```