analog.c: sr_analog_to_float(): Support for receiving double values.

analog.c: sr_analog_to_float(): Support for receiving double values.

Unbreak the conversion of input data in the floating point format when
the input data's format does not match the host's internal presentation,
thus grabbing raw data cannot be done.

Also accept the input data in double precision format which was not
supported before. Check for unsupported unit size values and emit an
error message similar to the integer code path.

This is motivated by bug #779 but does not resolve it. Adjusts common
conversion support, but sticks with single precision return data type
for API compatibility.