Optimize the fit gradients

[HanatoK]

• Save the unrotated positions. This can avoid rotating the positions again in calc_fit_forces_impl;
• Determine dl, dq or ds by compile-time template options;
• Only call calc_fit_gradients when atom groups have explicit gradients;
• Simplify and inline cvm::quaternion::position_derivative_inner.

This PR should be only marked as ready after #713 is merged.