Added get_filters function; Fixed register

[t20100]

This PR adds a get_filters function to retrieve filter classes from names or IDs.
By default is returns all filter classes.
It accepts a special argument "registered" and then returns classes of all currently registered filters.

The register function was also extended to supports IDs.
The FILTERS constant was removed from the documentation (not deprecated though): one can get filter classes with get_filters and then retrieve the filter_id from the filter classes.

A filter_name class attribute was also added to filter classes so one can get it from a class and it is shown in the documentation along with the classes.
Question: Is it really worth adding this filter_name which is specific to hdf5plugin to the API or not?

@rayosborn would that fits your need?

This PR also fixes a bug in register when one filter fails to register, the other are skipped: 58c56eb

closes #207

[rayosborn]

Thank you for this. It goes much further than I was proposing and the registered option in the get_fllters function looks especially useful.

[vasole]

Great work! (as usual)