Fix derivatives

[jtravs]

Maths.derivative needs more systematic analysis. Either an alternative approach, or at least a full systematic test of the correct FiniteDifference settings. We could also use an alternative such as spline derivatives.

See some discussion at the end of #99

[jtravs]

Also see: JuliaDiff/FiniteDifferences.jl#65
And references issue.

[chrisbrahms]

When this is fixed, the change from frame_vel to β1 introduced in #112 should be undone.