RascalC lesser expansion: covariance of 2PCF Legendre multipoles projected from µ bins (with jackknives)

This version features a new mode (LEGENDRE_MIX), corresponding to 2PCF Legendre multipoles not accumulated directly, but projected from many µ bins (as e.g. in pycorr). Usage of µ bins also allows to predict jackknife covariance more easily in the same run. (The disconnected term in the jackknife covariance is currently omitted in this mode, it has been very small in practice.) The corresponding documentation version is available on ReadTheDocs.