This feature allows us to plot non-state variables.

Currently, state variables are able to be retrieved from the ODESolution object, and therefore plotted, because they are passed in as initial conditions to the solver. However the variables specified for plotting are stored in a separate field, plotVars, and therefore may not have initial conditions.

The design decision made in this draft PR intends to also endow variables in plotVars with a default initial condition called "Zeros." The initial_conditions function dispatches on Zeroes initial condition type, returning zeros(geometry.dualmesh[:point]).

At the time of opening this draft PR, all variables in the uuid2symb mapping are given Zeroes ICs if they are not specified in the initialConditions field in the JSON.

My question is

1. should AlgebraicJuliaService restrict giving ICs to just symbols in either initial conditions (ic_specs) or specified to be plotted, or should every variable be given ICs?
2. Is giving a default IC always valid? For instance if we give Zeroes, what if the model has a singularity at 0?
3. Are there ways of extracting the trajectories of variables where ICs are not provided? I've checked ODESolution source and I'm doubtful.