dissimilarity.py missing from methods folder

[FanwangM]

Discussed in https://github.com/theochem/Selector/discussions/207

^{Originally posted by JackyZzZz May 15, 2024} I noticed that we need to import dissimilarity.py from methods in the Running Dissimilarity Algorithms example in the documentation. However, it seems like this file is missing from the methods folder.

P.S: I think it is renamed to distance.py, so we should probably update the documentation.

Add-on:

• the first parameter of select function in base.py is X, not arr in the documentation

I will do a cleaning up before more actual code changes.