Reduce number of parameters needed to configure PA levels

[vincentvanhees]

There seems no value in being able to use different thresholds for MVPA between part 2 and 5. Therefore:

• Use threshold.mod as value for mvpathreshold, if mvpathreshold is not provided.
• Replace mvpathreshold in documentation by threshold.mod.
• Use boutcriter.mvpa as value for boutcriter, if boutcriter is not provided.
• Replace boutcriter in documentation by boutcriter.mvpa.
• Correct parameter documentation for mvpadur to list c(1, 5, 10) as default because 10 is not the default.
• Double check that other parameter defaults are correctly listed.
• Document parameter bout.metric even though it has been deprecated.