Added records and protocol for analogy, propn nets

Something broken about the ns's still.
Fixed bug loading popco.core.nn in acme.clj

Problem was that the import statement has to come after
the require.  import has to be able to see a prior Clojure reference.
Renamed record types *Stru to *
Deleted posify, negify defns.
Added popco.core.nn to :use
Reorg make-nn-[stru-]core vs make-analogy-net-stru

What's now make-nn-core used to create the zero matrices
:neg-wt-mat and :pos-wt-mat, in addition to the node-to-index
map and the index-to-node vector.  Now it only does the last
two things, and creation of the zero matrices before being
filled (sparsely) with weights is now in make-analogy-net-stru.

This way, make-nn-core can be used for propn networks as well.
Misc renamings
Moved make-nn-core from acme.clj to nn.clj

Also moved make-id-to-idx-map, which is used (only) by make-nn-core.

make-nn-core creates a map containing the two index/node maps that
are needed by both analogy networks and proposition networks, so
it belongs in nn.clj.
Moved neural net source to new dir src/popco/nn

Also added some README.md's.
Renamed src/popco/nn/acme.* to analogy.*

This will make it easy to see that analogy.clj is parallel to
proposition.clj, which will contain functions specific to creating
proposition networks.
Added propn.clj, stub for propn-net creation fns