Use `PINT` or `IndexToken` in our code documentation?

[clearloop]

Issue summary

When people talk about PINT, people regard PINT as the token/currency of this project as our wishes.

Currently, we are using a namespace IndexToken in our code (docs) which is standard in development but easy to make the concepts mixed up for developers/contributors —— We already have too many concepts about tokens in PINT, it is necessary to simplify them.

I'm thinking that we can:

1. Use one namespace only, replace IndexToken with PINT in the code.
2. Keep two namespaces, doc the IndexToken that it is PINT we are commonly talking about.

[clearloop]

#235 (comment)

Re-open when needed