input/csv: add support for analog input data

Extend the CSV input module which was strictly limited to logic data so
far. Add support for analog data types. Implement the 'a' column format,
and feed analog data to the session bus.

This implementation feeds data of individual analog channels to the
session bus in separate packets each. This approach was found to work
most reliably, not all recipients support the submission of multiple
samples for multiple channels in a single packet.

A fixed 'digits' value is used. This needs to get addressed later.

Local experiments suggest that the 'double' data type for analog data
can result in erroneous visual presentation (observed with sigrok-cli).
Use 'float' for now, until the issue is understood and got fixed.
Support for double is prepared internally and is easily enabled.