cpp: bioformats: FormatReader series/index setting methods are const

While not const in reality, these are treatedly as effectively const
in order to allow for their use in const methods.
cpp: bioformats: series/index state is mutable

This allows for state changes while const.
cpp: bioformats: Add SaveSeries sentry to FormatReader

For use by any class implementing FormatReader, with methods
which need to modify reader series/index state.
wip4