Update needed for vii_l1b_nc reader to match a change to the Test Data and processor

[ColinDuff]

The VII L1b test data and processor is being updated to fix one of the variable names in the calibration data

Currently the reader will use the following to access the data, in the init method

self._integrated_solar_irradiance = self['data/calibration_data/Band_averaged_solar_irradiance'].values

but this name will now change to band_averaged_solar_irradiance

Read in the calibration data correctly

The above line of code will fail due to the case change in the variable name

Environment Info:
Linux
V0.25 - but i checked the main branch and the erroneous code is still there

[ColinDuff]

you can assign this issue to me, Colin Duff

[ColinDuff]

The new version of the test data should be ready around the end of the year so I will coordinate this reader change to match the test data update