Support Catalog Alignments of more than 2 catalogs

In the recent work on Association Tables, an alignment between the left catalog, alignment table, and right catalog was needed, but current alignment only supports aligning 2 catalogs. This could also come up in other situations such as crossmatch algorithms that work on more than 2 catalogs. The alignment class should be extended to support arbitrary numbers of catalogs aligned together, even if the alignment algorithm does them pairwise. This would also be a good time to work on #73, since the pixel_mapping can be difficult to work with in cases like this.

Start by locating the alignment class and the recent Association Tables work that requires a left catalog, alignment table, and right catalog. Trace how pixel_mapping currently represents two-catalog alignments, and review issue #73 for the related usability concern. Done means arbitrary numbers of catalogs can be aligned, including pairwise alignment algorithms, with coverage for the stated crossmatch and association-table cases.

Written by the indexing model from the issue text.