Zipfiles are written using .json extensions instead of .pas

[dbrakenhoff]

This means an unpacked zip cannot be read by PasConnector only because the series files have the extension .json instead of .pas.

Suggested change is to use .pas extensions everywhere. Add an option in from_zip() to use json extension for reading older zip files.

This also means we can shortcircuit from_zip for PasConnector, since it doesn't need to actually read/write the files, it only needs to unpack the zipfile into the current pastastore directory.