Streamline digitizer functions

@lleguy reported an error with this line:

run, ds = tb.load(8196, 181, fields=["XRD_MCP_SMALLraw"])

which showed that several updates of the digitizer functions were necessary.

• Remove hard-coding of bunch pattern in some functions
• Get the digitizer type based on classId. This solves the issue above, which occurred because of a wrong assignment of digitizer type based on the name.
• Update mnemonics to allow digitizer peak extraction
• Simplify get_digitizer_peaks by only allowing the processing of one mnemonic at a time.
• Simplify arguments of get_peaks() function, disable loading of data and of bunch pattern table: these should be done prior to this function.