Metadata input from "SDG Metadata" repositories

[brockfanning]

This allows the input of metadata from a source structured according to the SDG Metadata project. This structure can be described as:

• One main folder containing multiple subfolders
• Each subfolder is a language code
• Inside language folders are YAML files named according to the indicator id (eg 1-1-1.yml)

In other words, this is nearly identical to the InputYamlMeta class which we already have, except for these differences:

1. In this new class, the default language is in a subfolder just like the other folders. By contrast, in InputYamlMeta, the default language is in the main folder.
2. This new class allows reading directly from a Git repository, in additional to a local folder.

[brockfanning]

@LucyGwilliamAdmin This is ready for review. This can be seen in use here. With that usage, this class is reading in metadata from folders which are managed by Weblate, allowing Weblate to act as a sort-of "metadata editor" for the non-default languages.

[LucyGwilliamAdmin]

@brockfanning does this require any further testing from me? or is what you've done with Cambodia suffice?

[brockfanning]

@LucyGwilliamAdmin No I don't think you need to test it, since it's working in that case.