read_attribute: translate DATA_TYPE ci2 to float32 for roipac

[yunjunz]

Description of proposed changes

• utils.readfile.read_attribute(): translate DATA_TYPE value "ci2" to "float32" for roipac products. This is needed because various bugfixes in readfile and geocode for gamma products #1044 removed the hardwired value for roipac, and DATA_TYPE = CI2 exists in the KujuAlosAT422F650 example dataset (I do not remember how is this metadata saved here, as this metadata does not exist in ROI_PAC).

• tests/smallbaselineApp: more comments on example usage

Reminders

• Pass Pre-commit check (green)
• Pass Codacy code review (green)
• Pass Circle CI test (green)
• If modifying functionality, describe changes to function behavior and arguments in a comment below the function declaration.