Adjust trunk size field to enable tracking of growth rates

[dboyer]

Trunk size field currently can contain only one value. It would be useful to adjust this field so that users can enter a trunk size (diameter/circumference) and tag it with a date. They could then add another trunk size and a different date in the future to enable tracking of growth rates for the tree.

Note - users should not be able to set a date in the future as this should reflect existing trunk size, not projected future trunk size.