Refactor IO to use laspy

[rmsare]

Right now, write_pdal and write_las use temporary files to save a numpy array as a LAS file. This is bad because

• The temp file may be huge, and take a long time to write
• This doesn't keep any spatial reference information

Instead, write_las should use laspy's File object,to write points and copy a header. This means that read_las should also use laspy, and return a header to copy.