Write interleaved if RGB

All RGB data is assumed to come back as though stored as an
itk::RGBPixel type, and thus interleaved. Therefore when writing RGB
images, the writer should always be set to interleaved.
Corrected writer handshaking

The Write method was handshaking one too many times, leaving bytes in
the buffer.

Note that endCommand was updated to print a single endline again, to
avoid putting an excess of bytes on the buffer (in cases where the pipe
length may land between a 2nd and 3rd newline).

Also updated the handshaking method to clarifiy if it is reading more or
finished reading the bytes for a given plane.