[AGIPD] FIX: Always convert bias_voltage to integer

SPB and MID have the bias voltage stored as float and not integer as HED and as the function expected to return.

As the Bias Voltage precision is not import, always convert to INT in get_bias_voltage.

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• Bug fix (non-breaking change which fixes an issue)

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@danilevc @kluyvert @hammerd

assigned to @ahmedk

I remember Jola talking about this and how the precision made it difficult to compare with previous constants, thus showing no comparison of previous constants in the dark reports.
LGTM, thank you!

merged

resolved all threads

mentioned in commit d23db4a4

changed milestone to %3.5.4