ir_irmp: touch up the IRMP based decoder implementation (Python side)

Address Python and sigrok project coding style nits in the IRMP based
decoder for infrared signals. Re-add the attribution which was lost in a
previous copy and forgotten in the recent submission. Eliminate camel
case identifiers and adjust to the simplified Python binding API. Drop
remaining diagnostics from development and dead code (unused carrier
detection). Reword the boilerplate text to match other decoders. Avoid
Python f-strings since they are not portable. Prefer slightly less
cryptic variable names in the construction of annotation texts. Defer
the creation of the library instance until actual decoder use. Start
inspecting the input data at the very first samples in the input stream.