Implement LArPix converters

[peter-madigan]

Add a larpix_converters module to handle read and writes to HDF5 files / Dan's serial file / ROOT files.

[peter-madigan]

HDF5 analysis file converter is a start, but we still probably want:

• Raw serial file converter (do away with the dat2h5.py script)
• JSON calibration file converter
• ROOT file converter (for those who must use root)