Feat/15 support in1 policy number

[NiklasMM]

First commit adds support for item assignments, so content can be assigned to segments using their index:

message.in1[10] = "foo"

This is useful when writing tests.

The second commit adds support for index override in cell definition. This means that when defining cells for a specific segments in hl7_segments.py, you don't have to write down all cells until you get to the one you're interested in.

Example:

segment_maps = {
    'FOO': [
        make_cell_type('bar'),
        make_cell_type('bar'),
        make_cell_type('bam', index=12),
   ]
}

Before this change, you'd have to write down all 12 cells.

Finally, the last commit uses this to define policy_number for IN1 segments.