translate pruning zones to calendar months

[emillipede]

part of #38

SM public works breaks the city into numbered pruning zones. the numbered pruning zones correspond to thirds of the year - each zone is pruned in a defined 3 month period.

we can specify "Pruning scheduled for May - June 2020" rather than documenting the pruning schedule as: "Pruning Year: 2019-2020, Pruning Zone: 3"

eg. for the pruning year 2019-2020, the three zones would be:

July - October 2019
November 2019 - February 2020
March - June 2020

[captainrandom]

@emillipede I have no clue what this means. Could you help me with this?