MAVExplorer: allow arbitrary expressions to be plotted on the map

[tridge]

this allows mavextra to construct new mappable objects with Lat and Lng fields, allowing for derived positions from logs

this also generalises and simplifies the handling of instances

for example:

this uses the following mavextra function:

def pos_offset(GPS,offs_N,offs_E):
    latlon = mp_util.gps_offset(GPS.Lat,GPS.Lng, offs_E, offs_N)
    return PosObj(latlon[0],latlon[1],GPS.Alt)

[peterbarker]

LGTM