Min Step Size Bug

[jrenaud90]

During the original implementation of CyRK I misunderstood how the minimum step size check was supposed to work. Scipy defines the minimum step size based on the actual value of time,

import numpy as np
min_step = 10 * np.abs(np.nextafter(t, direction * np.inf) - t)

whereas CyRK originally had a constant step size based off of the floating point error,
defines the minimum step size based on the actual value of time,

import numpy as np
min_step = 10 * np.finfo(np.float64).eps

This latter approach works for when t is close to zero, but is not accurate when t is far from zero since the number of floats between numbers decreases at higher values.

Since min_step is used to set the step_size from becoming too small during integration - having an incorrect value could lead to incorrect or unexpected behavior.

This bug affects both the cyrk_ode and nbrk_ode.

[jrenaud90]

Closed by PR #25 and v0.5.0