Given a string s, write a method (function) that will return true if its a valid single integer or floating number or false if its not.
Valid examples, should return true:
isDigit("3")
isDigit(" 3 ")
isDigit("-3.23")
should return false:
isDigit("3-4")
isDigit(" 3 5")
isDigit("3 5")
isDigit("zero")