Prepare for graph work by importing igraph package
initial implementation of ngraph class (ngraph, as.ngraph, xymatrix(<-).graph)

* general way to represent connectivity of neuron
* depends on igraph
* can also store points as graph attribute
fix and test for as.ngraph.dataframe method

* more tests can be ported from AnalysisSuite in due course
make sure that graph actually produces objects with ngraph class
add graph.nodes/rootpoints/branchpoints
fix rootpoints/startpoints/branchpoints - switch to as.ngraph for default method

* not as.igraph
exercise rootpoints functionality
STY: final newline
more tests for graph.nodes

* noticed an error in TKC8R
  (and likely a number of other neurons in Cell07PNs)
some namespace fixes
rename test-graph to test-graph-nodes for consistency
verbatim copy of graph->neuron code from AnalysisSuite
Add neuron() constructor function

* and tidy is.neuron, as.neuron
Add as.neuron generic and methods for igraph and dataframe
replace graph2seglist with as.seglist generic + method for igraph objects
fix generation of empty seglists
rename graph.R to seglist.R

* since that is what it contains
Full set of as.seglist tests brought over from AnalysisSuite
more tests for graph.nodes - some still failing
Add seglist2swc (formerly RecalculateSWCData)

* will use this to go from SubTrees to swc (and then perhaps to graph)