Can't add a `CalPoint` to a `SensitivityList`

[ScottSoren]

A normal workflow in ixdat with siq is:

from ixdat import plugins, Measurement

plugins.activate_siq()
meas = Measurement.read("my_data.tsv")

cal_1 = meas.siq_calibration_method_1(...)
cal_2 = meas.siq_calibration_method_2(...)
cal_3 = meas.siq_calibration_method_3(...)

calibration = cal_1 + cal_2 + cal_3   # raises an Error! :( 

Where cal_1, cal_2, and cal_3 are all CalPoints.

The error is TypeError: SensitivityList.__init__() got an unexpected keyword argument 'sensitivity_list'
It arises because the + operator is implemented such that it works for Calibration and CalPoint but not for SensitivityList and CalPoint.

Workarounds:

For the above example, you could directly define the Calibration:

calibration = plugins.siq.Calibration(cal_list=[cal_1, cal_2, cal_3])

You also convert the SensitivityList to a Calibration before adding additional CalPoints:

calibration_partial = cal_1 + cal_2
calibration_partial = plugins.siq.Calibration(cal_list=calibration_partial.sf_list)  
calibration = calibration_partial + cal_3  # works now! 

However, neither of these feel like a smooth integration of ixdat and siq.

Ways to fix it:

(1) Make sure that addition of two CalPoints results in a Calibration, not a SensitivityList
(2) Modify SensitivityList.__add__ such that addition of a CalPoint to a SensitivityList returns a Calibration
(3) This raises the question of whether SensitivityList, which is a base class for Calibration, serves a purpose. It arose out of the need to make siq more modular in its early days (and conceptually separate the calibration and quantification workflows). But the separation is counterintuitive in some cases.