Update Cheetah's RFIM implementation of IQRM to the latest formulation of the algorithm

IQRM is a recently published RFI mitigation method for high time resolution radio data. An initial implementation of IQRM (before its publication) was added in Cheetah in 2020 in the RFIM module, but the algorithm itself has received significant improvements since then. The Cheetah implementation is now obsolete. We will update it to match the algorithm formulation in the reference paper, which has fewer corner cases and is expected to run faster: https://arxiv.org/abs/2108.12434