Added get_config() function to retrieve information

[t20100]

This PR:

• adds a debug log when successfully registering a filter.
• adds and document a get_config() function with provides access to both build-time options and runtime registered filters.

Previously hdf5plugin.config was available (and still is) to provide build time config but it is not documented.

I hesitated between providing a dedicated get_registered_filters() function and providing a more general get_config() one as this PR proposes. I chose the later to avoid having too many functions for accessing debug/more advanced information. Any other opinion? Better naming?

closes #185

[vasole]

Why is the information about the constants missing?

Well, I find it somewhere else. Probably it is somehow fetched from __init__.py instead of being explicitly written in the .rst file

[vasole]

The situation is:

• we need to know how hdf5plugin was configured
• we need to know what filters have been registered
• we need to know what registered filters come form hdf5plugin and which ones do not

[t20100]

get_config() provides:

• how hdf5plugin was configured
• what registered filters come form hdf5plugin

To me the last point "what filters have been registered" is in a wider scope than hdf5plugin and would be better suited in h5py

[t20100]

Yes, the constants are now documented with autodoc to avoid duplication between the docstring and the rst.

[vasole]

I agree it is a more generic functionality to retrieve all filters.

However, in the case we have skipped a filter because it was already loaded, I think we should somehow let the user retrieve that information without having to toggle the logging level and (perhaps) restart again.

@aragilar suggested to provide hdf5plugin loaded filters and used path (obviously if we load them we use hdf5plugin.PLUGIN_PATH) but if we do not load it because already there then the path is unknown...

[vasole]

Basically I am suggesting to add yield name, ("unknown", "unknown") after _logger.info("%s filter already loaded, skip it.", name)

[t20100]

Yes I agree.

As it is, one can already find this information by checking both embedded_filters and registered_filters, but all this information can be included in registered_filters.

[t20100]

I added a status message to the registered_filters:
To provide something like this:

In [1]: import hdf5plugin

In [2]: hdf5plugin.get_config().registered_filters
Out[2]: 
{'blosc': FilterStatus(status='Missing: not embedded in hdf5plugin', filename=None),
 'bshuf': FilterStatus(status='Missing: not embedded in hdf5plugin', filename=None),
 'bzip2': FilterStatus(status='Missing: not embedded in hdf5plugin', filename=None),
 'lz4': FilterStatus(status='Registered', filename='/hdf5plugin/plugins/libh5lz4.dylib'),
 'zfp': FilterStatus(status='Missing: not embedded in hdf5plugin', filename=None),
 'zstd': FilterStatus(status='Missing: not embedded in hdf5plugin', filename=None),
 'fcidecomp': FilterStatus(status='Missing: not embedded in hdf5plugin', filename=None)}

but it's maybe too much, this is already in the logging message, maybe better to keep it simple.

[vasole]

I think the requests are already fulfilled. The rest is just a question of taste.