Start with is.vaa3draw function

* works with Hanchuan's test data
fix bug when raw data is passed to is.vaa3draw
initial implementation of read.vaa3draw

* works for 3d data
* problems with 4d data since im3d does not have a model for that
* ReadData=FALSE doesn't work yet
Complete and document functional version of read.vaa3draw
teach im3d to handle image data without a proper bounding box

* we’re going to need this to read raw images (without spatial calibration)
  as im3d objects
remove a test for null bounding box

* now that bounding box will be set to dims if no other information is
  available.
Fix vaa3d reader for int64 dims and returned data
implement vaa3d raw support in read.im3d
Add tests for reading 1 and 2 channel v3draw data
one more test for a 2 channel single slice image

* should be possible to read this
Merge branch 'feature/im3d-hpengraw'

* closes #110
* feature/im3d-hpengraw:
  one more test for a 2 channel single slice image
  Add tests for reading 1 and 2 channel v3draw data
  implement vaa3d raw support in read.im3d
  Fix vaa3d reader for int64 dims and returned data
  remove a test for null bounding box
  teach im3d to handle image data without a proper bounding box
  Complete and document functional version of read.vaa3draw
  initial implementation of read.vaa3draw
  fix bug when raw data is passed to is.vaa3draw
  Start with is.vaa3draw function