GSF-17 Correct parsing logic for common expressions in TemplatedExpressionParser

[StephenCWills]

This PR fixes an issue where a template with a common expression that has square brackets nested within it would not parse correctly.

I tested using the program below. Before the changes in this PR, I received the following error.

ExpressionEvaluator.Parser.ExpressionParseException: 'Error parsing token '7' at line 1 char 26'

The expected result is DAMP.MODE50.!.MAG.

using System;
using System.Collections.Generic;
using GSF.Parsing;

namespace TemplatedExpressionParserTest
{
    internal class Program
    {
        static void Main(string[] args)
        {
            TemplatedExpressionParser parser = new TemplatedExpressionParser();
            parser.TemplatedExpression = @"[?{Output}=ModeShapeMagnitude[DAMP.MODE{ModeNum}.eval{'{Input.PointTag}'[7]}.MAG]]";

            Dictionary<string, string> substitutions = new Dictionary<string, string>()
            {
                { "{Output}", "ModeShapeMagnitude" },
                { "{ModeNum}", "50" },
                { "{Input.PointTag}", "GPA_PDC!SHELBY:FREQ" }
            };

            string result = parser.Execute(substitutions);
            Console.WriteLine(result);
        }
    }
}

[ritchiecarroll]

Minor change requested.