jefferis/rcatmaid: catmaid R package v0.9

This version is synced with the upcoming release of nat v1.9.0 which streamlines some functionality but has some breaking changes in behaviour. It must be used with nat >=1.8.12.900 from github.

• switch to Ops.catmaidneuron to match nat 1.9.0 in prep (and add version requirement)
• Simple search for skids with exact annotation match (#89) Now you can do: catmaid_skids("ORN") or read.neurons.catmaid("ORN")
• Fix bug in catmaid_skids error message when looking for one skid (#90)