Drop internal ecCodes bindings and switch to eccodes-python

[alexamici]

The internal API is ready and you can test it by setting the environment variable CFGRIB_USE_EXTERNAL_ECCODES_BINDINGS to ON, for example:

$ pip install eccodes-python>=0.9.2
$ CFGRIB_USE_EXTERNAL_ECCODES_BINDINGS=ON ipython

[cdibble]

Does it need to be CFGRIB_USE_EXTERNAL_ECCODES_BINDINGS=1 ?

given:

try:
    # select between using the external ecCodes bindings or the internal implementation
    if int(os.environ.get('CFGRIB_USE_EXTERNAL_ECCODES_BINDINGS', '0')):
        import eccodes
    else:
        from . import bindings as eccodes

from messages.py