erasure-code: add chunk size calculation helpers

With the current erasure code interface, figuring out the size of a
chunk and the number of chunks for a given object can only be achieved
by calling the encode() method on an actual buffer.

The get_chunk_count, get_data_chunk_count and get_chunk_size helpers are
added and documented.

Signed-off-by: Loic Dachary <loic@dachary.org>
erasure-code: refactor the example to use chunk size helpers

The get_chunk_size and get_chunk_count methods are implemented using the
code found in the encode() method. The encode method is modified to use
the chunk size helpers.

The unit tests are modified to use the helpers.

Signed-off-by: Loic Dachary <loic@dachary.org>
erasure-code: jerasure implementation of chunk size helpers

The encode methode uses the get_chunk_size method which is a translation
of the code it previously used for the same purpose. The chunk_count and
chunk_data_count methods return k+m and k respectively. But that will
no longer be the case after local parity is implemented.

Signed-off-by: Loic Dachary <loic@dachary.org>