Add tester for numerical calculation of PI

Calculate PI using float, double and Real and compare the error and performance.

Traditional floats have a point (hardware dependent) where the error starts to behave unpredictably
rather than decreasing with number of intervals - due to rounding/precision errors.
Can see that doubles are better than single precision floats but they still do it.

An infinite precision Real would not have these problems (hah!)
We can at least aim to do better than double.

Based on PHYS CQM/pprog courses from 2012.
Should probably find a literature reference for this.