Added function validators to ensure only valid values are computed

Check all symbols are unique & migrate to .NET 7

The former checks ensured no duplicate symbols within each category, but did not cross check them with other categories.

Added checks to ensure all symbols are unique

Fixed case sensitivity on custom operators and functions

Fixed scientific notation formatting issue from functions

Merge negitive fix

Refactor token value to remove casting to object

Added support for functions

Added support for constants

Updated namespace to reflect the nuget package