Fix/string set<modified string>

This patch add the following expected features:
1) you can assign modString = hostString; [make the modified string point to a new host]
2) you can assign a concatdirect-stringset of hoststrings to a concatdirect-stringset of modstring, which copies the limits and does 1) for the concat member
2) you can save and open modStrings, as well as stringsets of modstrings, currently implemented as a NOOP, but required for saving indexes over modStrings (among other things)
`_initStringSetLimits` seems superfluous here and in the other constructors.