Better type checking using the numbers module

[Jellevanderwerff]

Sometimes input validation is done using e.g. isinstance(input, (int, float, np.float64)) etc. Better change this to use the numbers module, e.g. isinstance(input, Integral)

[YannickJadoul]

Should be numbers.Real, to cover float and not just int

[Jellevanderwerff]

Let's reconsider this once we consider integrating mypy