str.isnumeric method to check if all characters are numeric

Returns true for all non-empty strings that only contain numeric characters.

```In [1]: 'a'.isnumeric() Out[1]: False

In [2]: ''.isnumeric() Out[2]: False

In [3]: '1'.isnumeric() Out[3]: True

In [4]: '123'.isnumeric() Out[4]: True ```