Transition battery_t to return const pointers instead of copies where possible

[brtietz]

battery_t::get_params() returns a copy of the entire params object, which includes copying lifetime arrays. This can be a major user of memory and decreases runtime to do frequent copying.

At the moment, all functions in ssc have been migrated to call out for their specific data, but it may still be helpful to make this return a const pointer for increased speed/memory. There are some potential downstream headaches with battery_t's copy constructors to watch out for in this migration.