Flux point method

Implements #50 .
Either do
```
from .. import flux_point
```
and use `flux_point._x_lafferty` etc everywhere,
or break the line like this:
```
from ..flux_point import (_x_lafferty, _integrate, _ydiff_excess_equals_expected, 
                                      compute_differential_flux_points, _energy_lafferty_power_law)
```