test that we get a warning for neurons without width data

* only source of these at the moment is flycircuit
Divide up lengthy neuron i/o tests into 3 separate contexts
expect a warning when we can't overwrite
suppress warning that immediately precedes a test error

* trying to clean up warnings and this one is superfluous
remove a few more warnings in tests
Fix left over temporary file in xformpoints.cmtkreg

* this had been there for a while and was a classic case of on.exit not having
  the add=TRUE argument
* closes #65
Ensure tests do not leave temp files behind

* in combination with 5d550082d4e21630a127cfd39ad1709a1f376e12 fixes #64