Add global_index property for creating unique line numbers

[yeu-degroot]

This PR adds two new properties which allow you to add unique line numbers to the outputted content:

• global_index
• global_index1

Example usage:

[gwrite.line_numbers]
document_start = """
N{global_index1:03d}0 (document_start)
N{global_index1:03d}1 (document_start, inserted line by the config)
"""
layer_join = "N{global_index1:03d}0 (layer_join)\n"
segment_first = "N{global_index1:03d}0 (segment_first)\n"
segment = "N{global_index1:03d}0 (segment)\n"
segment_last = "N{global_index1:03d}0 (segment_last)\n"
document_end = "N{global_index1:03d}0 (document_end)\n"

Example output:

N0010 (document_start)
N0011 (document_start, inserted line by the config)
N0020 (segment_first)
N0030 (segment)
N0040 (segment)
N0050 (segment)
N0060 (segment_last)
N0080 (segment_first)
N0090 (segment)
N0100 (segment)
N0110 (segment)
N0120 (segment_last)
N0140 (segment_first)
N0150 (segment_last)
N0170 (layer_join)
N0170 (segment_first)
N0180 (segment)
N0190 (segment)
N0200 (segment)
N0210 (segment_last)
N0230 (segment_first)
N0240 (segment_last)
N0260 (document_end)