Minor changes to code

- Changed the type of Expression's getAnswer() to double
- Switched the input letters for the menu so they are accurate
- Changed the type return of evaluate_expression() to double
- Modified evaluate_expression() to return NaN if syntax check fails
- Slightly modified the output of the menu to place syntax errors at the
right place
- Expanded the number of expressions that can be read from a file from
50 to 1000