Handle default ADC slope of 0. in new frameCPP versions
This PR fixes a failing test when running with the latest versions of ldas-tools-framecpp, in which FrAdcData structures are created with a DEFAULT_SLOPE of 0., with no way to change the slope from the Python bindings. This means that if you write a GWF from Python that contains ADC channels, the slope you read back is always 0.. The patch here is to replace a default of 0. with 1. if the bias is also at its default (0.).