WIP: introduce Hawc2io.ModifyHawc2, a Hawc2 result writer.

Introduce the class ModifyHawc2 (inherits from ReadHawc2), which can modify and write Hawc2 result files.

Channels can be added or removed from a result file, and written back into a HAWC2 format (either HAWC2_BINARY, HAWC2_ASCII).

Currently only supports writing to HAWC2_BINARY format (including writing the .sel file).

Tests have been included to ensure the writer can rewrite the time series data with no numerical losses.

To do before merge:

• Implement ModifyHawc2.WriteASCII(..)
• Implement ModifyHawc2.remove_channel(..)
• Anything else?