Support reading and writing surface normals

[Ephraim-Bryski]

This is a fix for #34 and #26.

Reading normals just required adding the fields to SUPPORTED_POINT_FIELDS (I went with float32, which matches libe57). I also replaced the file I was using to test reading unsupported point fields, now that normals are supported.

Writing normals required very similar code to adding color in write_scan_raw. However, it also requires adding the prefix 'nor'. I did a bit of digging, and the StructureNodeImpl::set checks the element name, including checking if the prefix is valid (text before the colon), i.e. added to a list of supported prefixes. The prefix can be added with ImageFileImpl::extensionsAdd, which was already included in the wrapper.