do not instantiate an exception in assert statement

as the exception is made an AssertionError anyhow.
[core.exceptions] remove unused CodimError
[grids.rect] remove visualize() method from RectGrid
[docs] update/add documentation
[la] remove the Communicable mixin

instead, add data property directly to VectorArray implementations
and let the implementor decide how to handle details (give warnings,
etc). To test whether a VectorArray supports communication, simply
use hasattr(v, 'data')
[docs] update/add documentation