Add support for halls position in the venue

[furest]

This PR adds support for reading a hall position from seatmap.json and adding it to ipplan.

New ipplan table :
hall_positions (name, x, y)

Expected seatmap.json format (as an item of the root "array" defined in the file):

{
    "hall": "Operations Center",
    "type": "hall_position",
    "x": 2,
    "y": 0
}