cal line subtraction

We need to work out how calibration line subtraction will be handled. Presumably we'll use the same pydarm calibration function we're using for the main strain, but applied to just against a narrow band around the calibration lines in a special CalLineTransform element, then subtracted from the strain.

The CalLineTransform element needs to:

• read in the cal line raw channel and the EPICS control channels
• determine which lines are active
• calibrate the line data