Algebra: Implement a parser for function arguments that are not enclosed by parentheses

[avighnac]

  if (s[after_pow] != '(')
    {
      throw std::runtime_error ("Error: Function contents have to be enclosed "
                                "in parentheses.");
    }

Instead of throwing a runtime error, try interpreting it. For example:

• sinx = sin(x)
• log2 = log(2)
• sqrt54 = sqrt(54)
• tan2x= tan(2x)
• cbrtx+4 = cbrt(x) (+ 4, but this part is ignored)