Add option to "or" overlapping pixels when concatenating integer maps

[erykoff]

Currently, when concatenating maps with cat_healsparse_files the only option is to overwrite values when two maps have the same pixel defined. When concatenating integer mask maps, it is often useful to be able to "or" together the overlaps (which allows, for example, efficient parallelization of rendering of star masks).

[fjaviersanchez]

Closed by #88