use first endpoint rather than first point when no origin specified

* usually the first point is an endpoint but sometimes not and using
  an endpoint as the origin always makes more sense.
* NB first means first in raw vertex id sequence
Add segmentgraph() to generate a simplified segment graph
Give NrrdProject cropmin and cropmax options

* and add docs and a test
Switch to sys.source in startup - loads into .GlobalEnv or nat.as namespace

* this change should have no effect on the traditional style of
  AnalysisSuite startup ie
    source("~/projects/AnalysisSuite/R/Code/Startup.R")
* but should also allow the objects to be loaded into the 
  nat.as namespace when AnalysisSuite is loaded through 
  that wrapper package.