Add compression helper functions #33

Create 2 helper functions (bitshuffle_options(nelem=0, lz4=False) and lz4_options(nbytes=0) for the options to provide to bitshuffle and lz4 filters as it already done for blosc with blosc_options.
This should replaces the variable BSHUF_LZ4_OPTS.

Documentation and tests needs to be updated accordingly.