 -2, -3.3) or unsigned (eg: 1, 2, 3.3) integer or double
    or use the "scientific notation" (eg: 1e5).

    *Examples:*

    >>> is_number('42') # returns true
    >>> is_number('19.99') # returns true
    >>> is_number('-9.12') # returns true
    >>> is_number('1e3') # returns true
    >>> is_number('1 2 3') # returns false

    :param input_string: String to check
    :type input_string: str
    :return: True if the string represents a number, false otherwise
    N)