read.im3d now adds input file as an attribute

* closes #131
doc: document core attributes of im3d
fix example of reading vaa3d raw image

should use read.im3d not the read.vaa3draw.im3d function
Teach read.im3d to read amiramesh files without standard extension
test reading of amiramesh file without standard extension
Bump version to 1.5.5
docs: typo
fix: writing neurons in subdirs when df is null

* when neuronlist did not have an attached data.frame but you set subdir=
  expression to be evaluated in the current environment (rather than in the
  context of the attached data frame) then there was an error
* +test
* closes #133
give xform.neuronlist a subset argument

* so that it only xforms a subset of the neurons 
  in the original list.
* sample use case would be mirroring some members 
  of a neuronlist onto a canonical side of the brain.
* closes #134
give mirror.neuronlist a subset argument as well

* and make sure it is used