The MIMO classes now store the channel matrix.

The channel must be passed either when the object is created of by
calling the set_channel_matrix method. Furthermore the encode and decode
method do not receive the channel matrix anymore.
Fix a bug in 'gmd' where the inputs were changed
Add 'reshape_order' argument in the Blast class

The _encode and _decode methods in the Blast class now receive a
reshape_order argument. This was added so that subclasses of Blast can
use different reshape order.
Implement the GMDMimo class