Allow map deme sizes to represent different properties

[jameshadfield]

Currently map deme sizes are represented by visible* tips. For many datasets there are other quantities that may be desirable to use for this, e.g. ABX, titer values. Furthermore, this allows epi data to be integrated into the tree as weights for each tip.

This issue proposes allowing these as user options via the sidebar, with the current "visible-tip-count" being the default. Available values would be colorBys with numeric values.

Would best be done in conjunction with #280.

[*] the "visibility" of tips is not the same as that used for the tree, and this should be changed to represent the visibility as displayed on the tree.