Fix issue #2: t=0 -> t_i

[luciansmith]

Fix for new release of l3v2; new language for what 'initial time' means.

[fbergmann]

@luciansmith I think it should be t_0 rather than t_i, since we explicitly want to mention the initial time.

[luciansmith]

@fbergmann I'm not sure I understand. t_i is the initial time that we're explicitly talking about. In the current spec, we use 't_i' to talk about the earliest time that you have to calculate if there are delays, but in my revised version, I use t_i to mean the initial time, and just talk about 't < t_i' for the times before that. Will that work?

[dweindl]

Thanks for addressing this issue in SBML, @luciansmith ! I agree with @fbergmann that $t_0$ might be preferable. $t_i$ is correct, since it is defined as the start of the simulation time, but when just looking at any later section, $t_0$ might be clearer (or $t_{\mathrm{initial}}$, ...). When I see $t_i$, my first association would be that $i$ is some varying index, not "initial".