Rename block to blockMechanism, blockSynapse to blockingSynapse etc.
Made block and plasticity optional in blockingPlasticSynapse.

Also, removed PlasticSynapse (as it is just a blockingPlasticSynapse
where no block element is specified) and updated NMDA LEMS example to
show that a blockingPlasticSynapse with no plasticity element can
act as a replacement for nmdaSynapse.