Refactor subsystem builder methods that can be properties #347

Multiple different methods in SubsystemBuilderBase could be properties to simplify their use. This is backwards incompatible though generally worthwhile; could deprecate the existing methods.

    @property
    def get_mass_names(self):

so it can be accessed as BatteryBuilder.get_mass_names instead of BatteryBuilder.get_mass_names(), since no arguments are used

Originally posted by @crecine in #317 (comment)