Possibility of saving solution at irregularly spaced time intervals

Currently, `SaveSolutionCallback` accepts only either a fixed `interval` or a fixed `dt` parameter (i.e. outputting either at uniformly spaced timesteps or at uniformly spaced times).
From my experience it would be nice to have the option to pass in a list of output times which may be non-uniformly spaced (for example, in some reacting flow computations, a logarithmic timescale is really helpful as a lot of kinetics can slow down exponentially over time).

If there is interest in having such a feature (probably as an additional keyword in the `SaveSolutionCallback` initialization?) I could try my hand at implementing it.