Implement an option to count the frequency of node labels

[sthiele]

Similar to the option

    -l, --show-labelings <max-labelings>  

but

    -c, --count-labelings <max-labelings>  

Use a map to store/count labels instead of printing.
In the end print a statistic.

[sthiele]

I recommend that we create a branch based on this one https://github.com/bioasp/iggy/tree/next.

1 Adding a flag for counting

Here is where the command line options are defined.

iggy/src/bin/iggy.rs

Lines 64 to 70 in b4027e0

	/// Show max-labelings labelings, default is OFF, 0=all
	#[clap(short = 'l', long = "show-labelings")]
	max_labelings: Option<u32>,
	/// Show predictions
	#[clap(short = 'p', long)]
	show_predictions: bool,

Thinking about it. I think we should rename the option from --show-labelings to --enum-labelings where the default behavior is printing the labelings. Additional we add a flag --count which disables printing and turns counting on.

This is the library(crate) we use for command line parsing https://docs.rs/clap/3.0.0-beta.2/clap/index.html