Methods common to all representations
=====================================

Representations indicate the format of patterns in layers, and conversions
between representations govern the content of connections between layers.

description
-----------

	some_rep description

The description of `some_rep` additionally contains `Width`, and indication
of the typical (or compulsory) width of associated patterns.

find_conversion
---------------

	some_rep find_conversion source_rep

Return a conversion that converts patterns with representation `source_rep`
to patterns with representation `some_rep`.  If no current conversion
exists, attempt:

1. Use the `force_conversion` mechanism for the subtype of `some_rep`. Most
   notably, this applies to blocked\_representations, which will attempt to
   produce parallel\_conversions if the number of blocks match, or if one 
   end of the conversion has zero blocks, will change
   the number of blocks to match the opposite end.
2. Use any `conversion_solver` mechanisms that have been loaded.  Most notably,
   `conversion_chain` loads one of these to create a `conversion_chain`.