MathInterpreter

Mathematical expression interpreter for real-valued input.
Works only with real numbers and returns real numbers. Uses the Shunting
Yard Algorithm.

How to use:

Have the mathematical expression you want to solve stored in a string in infix notation

e.g.
std::string expr = "-12.4 + exp(sin(rad(68))) * log10(96)";
Create a MathInterpreter object and call init_with_expr() initialize the interpreter with the given input expression.

e.g.
```
 MathInterpreter inter; 
 inter.init_with_expr(expr);
```
Call calculate() to calculate the expression and save it in a double.

e.g.
double result = inter.calculate();

Have the mathematical expression you want to solve stored in a string in infix notation. Use dollar sign($) before and after variable names to mark variables.

e.g.
std::string expr = "-12.4 + exp(sin(rad($x$))) * log10($y$)"; //x and y are variables.
Create a MathInterpreter object and call init_with_expr() initialize the interpreter with the given input expression.

e.g.
```
 MathInterpreter inter; 
 inter.init_with_expr(expr);
```

Use set_value() to set values for each registered variable.

e.g.

 inter.set_value("x", 12.75);
 inter.set_value("y", 3.12);

Call calculate() to calculate the expression and save it in a double.

e.g.
double result = inter.calculate();

Function names can be all lowercase or all uppercase.
Pi is recognized automatically when entered as a variable.
e.g. sin(2*$pi$*5) or sin(2*$PI$*5)