Adding support for 3rd party packages providing marshallers as plugins

[frejanordsiek]

One major missing feature of hdf5storage has been that marshallers for any additional types had to be provided by the user by making a MarshallerCollection with the additional marshallers passed manually and then pass that to Options or through the keyword arguments of the various functions. This is enough to allow packages to use hdf5storage for their own types underneath the hood, but not for users to get access to the same marshallers when using hdf5storage directly.

Allowing other python packages to specify plugin marshallers via entrypoints in their setup.py makes it easier for users to read and write such types directly and makes it easier to extend hdf5storage.

[frejanordsiek]

Added initial support to master branch in commit 817da0b . There is still more work to be done to round it out and document it.