This would be useful to have so we could launch a new run with volume data from a previous run as numeric ID. We just need to have the proper functions of time to be able to do this, otherwise things will be discontinuous. There are two main ways I see to do this:

• Write a new option that takes in a volume file (or glob and assert the result is the same for all) and populate all the functions of time with that. Things to consider:
  • The glob is nice because then you don't have to worry about picking a specific volume file (even though 0 will probably always be there). It also has an added benefit that you're guaranteed all your volume files have the same functions of time (so long as you check that). Not strictly necessary, but could be good to have.
  • What to do when reading volume files without specific functions of time that are needed?
    • Error?
    • Set reasonable defaults? (like no rotation, or spherical shape map)
  • What to do with extra functions of time?
    • Error?
    • Ignore?
  • Option in *BCO.hpp files would change from the first to the second

struct TimeDependentMaps {
  using type =
      Options::Auto<bco::TimeDependentMapOptions, Options::AutoLabel::None>;
  static constexpr Options::String help = bco::TimeDependentMapOptions::help;
};

struct TimeDependentMaps {
  using type =
      Options::Auto<std::variant<bco::TimeDependentMapOptions, FromFile>,
                    Options::AutoLabel::None>;
  static constexpr Options::String help = bco::TimeDependentMapOptions::help;
};

• Same as above, except we do it on a FoT by FoT basis instead of just all at once.
  • More flexibility. Don't have to worry about missing/extra FoTs because it's up to the user to specify which ones should be read in from file.
  • More verbose code/input file than the first option. Would have to change each individual FoT option in TimeDependentMapOptions in BCOHelpers.hpp