CommandLineArgumentParser help output doesn't include plugin values

[cmnbroad]

Plugin descriptors have one or more string collection arguments that hold the plugins the user wants to enable (i.e, the read filter plugin descriptor has an argument for which read filters the user wants to use). The --help output for these arguments should include the list of possible values, which are dynamically discovered by the plugin system.

The bug is that the code assumes that such arguments are fields immediately within the descriptor class, which is myopic, and incorrect, since these are often embedded in a dynamically created argument collection object).