Added convinience methods to Sexp and Atom

[eytans]

This pull request adds concise type-checking and value extraction methods to the existing sexp parsing library. These additions are particularly useful when expecting a specific format of an s-expression. The changes include:

In the Atom struct:
    Added methods for checking if the atom is a string, integer, or float.
    Added methods for extracting the string, integer, or float value from the atom.

In the Sexp enum:
    Added methods for checking if the s-expression is an atom or a list.
    Added methods for extracting the atom or list from the s-expression.

These new methods provide a convenient way to validate and extract values from atoms within s-expressions, allowing for easier handling of expected formats. This enhances the usability and flexibility of the library when working with specific types of atoms within s-expressions.