Change order of derivative arrays

[langfzac]

The arrays of derivatives computed by NBG are ordered differently than the initial conditions. Namely, the masses come after the coordinates in the derivative arrays, but before the coordinates in the initial conditions.

@ericagol Do you have a preference for which direction the change occurs? Either will require some work and break existing scripts.

$[x_i, y_i, z_i, vx_i, vy_i, vz_i, m_i] \leftrightarrow [m_i, x_i, y_i, z_i, vx_i, vy_i, vz_i]$

[ericagol]

@langfzac I like having the masses come first.