add the basic prjn finding methods to AxonPrjns

[rcoreilly]

AxonPrjns has no useful methods other than Add, and everything has been moved to the layer (SendPrjnNameTry etc) which return emer.Prjn interface and are not super useful for actually operating on concrete axon.Prjn. The emer interface only has Try methods which require casting type in a separate step due to err return. very inconvenient.