New methods for interacting with metadata

[stuartlynn]

This PR

• Moves away from using the type based metadata system to the database / dataframe based one.
• It defines a new Metadata object which contains dataframes of each of the metadata tables.
• It defines metrics to read these per country from azure
• It defines a function to load and merge all countries in to a single metadata object.
• It defines methods on the metadata struct to find specific metrics
• It defines methods on the metadata struct to find specific geometries.

TODO

• [x] Figure out how to do fallback for year and geometry level if not explicitly specified ( default to latest year and smallest geometry level?)
• [ ] ~Figure out if we can implement From<Row> for MetricMetadata etc to get more structured data back from the dataframes~ Opening ticket #40 to perhaps attempt this in the future but it doesn't need to be part of this PR.
• [x] Decide on a unique metric id column (HXL seems like obvious one but I am not 100% sure if that will exist for all of the data we currently have (for example the US ACS columns)) ( we have decided to allow for specification of the metric using multiple columns with potential wildcarding)

[sgreenbury]

I think this is looking great! I've left a few small comments in the code.