Fast IP and stable IP

[tky823]

Summary

• FastIP: remove redundant operations in IP
• StableIP: Improve stability by ensuring the matrix is positive-(semi)definite.

[tky823]

In my naive implementation, FastIP is not faster than conventional IP.
Rather, FastIP is a little bit slower.

https://colab.research.google.com/github/tky823/ssspy/blob/cc398f53a16a99591ee65a8d236fd4f062209534/notebooks/ISSUE/AuxIVA-FastIP1.ipynb
https://colab.research.google.com/github/tky823/ssspy/blob/cc398f53a16a99591ee65a8d236fd4f062209534/notebooks/ISSUE/GaussILRMA-FastIP1.ipynb