the structure of Firestore database

[IanChen83]

Currently (as in PR #3 ) each sequence is store in one document, in the format of

{
  name: '<seq name>',
  values: [
    {
      timestamp: '<timestamp>',
      value: '<value>'
    },
    ...
  ]
}

and all documents live in a single collection. I think we can create different collections for development and production environments.

Any idea or comment about the format of sequences? Is there any other information we need to store?