BUFR package flavour added with optional eccodes dependency

[PennyHow]

• eccodes moved from an essential dependency to an optional dependency
• This flavour of pypromice can be installed with pip install pypromice[bufr]
• For the default pypromice distribution, the bufr functionality produces an import error if called and testing of this submodule is omitted when called in tests/unit/bufr_export
• pip install pypromice[all] flavour added also, so all future optional dependencies (e.g. the Google REST API dependencies) will be automatically tested with no need to update the pypromice flavour used