Retrieve split up JSON files

[gareth-j]

• Describe the changes implemented in this PR (Bug fix, feature, docs update, ...)
  This implements the dynamic retrieval of JSON files which hold each source (species, site, instrument, inlet...)

NOTE: This currently just uses a sample of the DECC data (CO2, CH4 and CO) and uses a temporary data repository (https://github.com/openghg/temp_data_dashboard). These will change before this PR is merged.

• Please check if the PR fulfils these requirements

• Closes Implement multi-file data retrieval #45
• [Tests added and passed] if fixing a bug or adding a new feature
• Documentation and tutorials updated/added
• Added an entry in the latest CHANGELOG.md file if fixing a bug or adding a new feature
• ESLINT passed

TODO

• Fix this.props.siteStructure is undefined on clicking on on Background
• Add tests
• Add in config file for setup of remote data repository instead of hardcoding URL

[gareth-j]

Most of the work is done. I've just encountered a nasty bug when clicking on the Background button in the sidebar. I'll take a look at this next week as I'm moving onto my other project today. I think the rest of the code is ready for review though.

[SutarPrasad]

Looks Good. One small suggestion was added as a comment.

[SutarPrasad]

An icon from here can be helpful:
https://mui.com/material-ui/material-icons/?query=loading

[cypressf]

I think chunking data makes sense if you've exhausted the potential for data compression, alternate data file formats, and reducing needless precision. Have you determined that even with those three approaches the total size is still not under 1MB? If so, ship it! If not, I'd encourage you to investigate those to see if you could keep the code simple and still load all the data at once.