Custom index tutorial

[e-marshall]

This PR would add a notebook demonstrating how to build a custom index to the tutorial. It is loosely based on the how to create a custom index page (which was very helpful in writing this) but is aimed to be a bit more explanatory.
it has the following structure:

• a bit of background and introduce the motivation for using custom indexes
• build a custom index for a simplified case of a 1d dataset where we'd like an index to handle a coordinate transform
  Steps to build index:
• build skeleton index with just from_variables()
• add create_variables(), sel() and transform
• add equals(), join() and reindex_like() to support alignment

If folks think this would be a worthwhile addition to the tutorial, I'd love to keep working on this and to incorporate any feedback and suggestions

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[scottyhq]

If folks think this would be a worthwhile addition to the tutorial, I'd love to keep working on this and to incorporate any feedback and suggestions

Absolutely this would be a great addition @e-marshall!