RascalC fixes and utility scripts for 2PCF covariance for s, µ bins and Legendre multipoles

This version is a large collection of bug fixes and utility scripts for v2.2, not introducing major new features like a new mode reserved for v2.3. Many corrections and improvements are in the documentation.

The extra convergence checks follow Rashkovetskyi et al 2023.

3PCF and power spectra functions remained largely unused and untested; most fixes concern the different 2PCF modes.