Parse a string and verify it is a float.
In the above diagram:
| DIGIT | → | the characters "0" thru "9" |
| Ee | → | the character "E" or "e" |
| EOS | → | end of string |
| PERIOD | → | the period character (".") |
| SIGN | → | the character "+" or "-" |
Note: The starting and ending spaces have been removed from the string.