Add specification for LB data file for phases skipped or identical

[lifflander]

What Needs to be Done?

We want to specify for every phase in the data file whether data is skipped or identical. This is related to issue #1935 which will allow phase output to be skipped for some outputs. The identical to previous is for data input where the mapping is the same as a previous phase and thus does not need to be output again.

{
	"metadata": {
		"rank": 0,
		"shared_node": {
			"id": 0,
			"num_nodes": 1,
			"rank": 0,
			"size": 4,
			"available_memory": 32768
		},
		"type": "LBDatafile"
                "phases": {
                  "count": 100,
                  "skipped": {
                    "list": [10, 13, 15],
                    "range": [[20, 25], [30, 35]]
                  }
                  "identical_to_previous": {
                    "list": [50, 53, 58],
                    "range": [[60, 70]]
                  }
                }
	},
	"phases": [

	]
}

*Edited to count

[lifflander]

@thearusable Let's actually rename the "length" field to "count" as suggested by @nlslatt