MGWR v2.1 for PC

1. Only allow Fixed Gaussian + Adaptive Bi-square kernel combination.
2. Report Gaussian kernel bandwidth * 2.45 (where the weight for the bandwidth drops below 0.95).
3. Add sum of weights for GWR and MGWR.
4. Add local_R2 for MGWR (calculated based on optimal GWR bandwidth)
5. Add degree of dependency for GWR and MGWR. For MGWR, there is also covariate-specific DoD_j calculated.
6. Add OLS residual in the output.
7. Add interval search for MGWR.