Context sort

[texodus]

• Modifies current sorting behavior in the presence of column pivots to sort by the top-level row header by default, rather than the leaf columns from left to right. This has the effect of preserving sort order of the same view without column-pivots. Fixes Sorting on a view with column pivots applied, should default to sorting on the row header value #216
• Modified perspective library default aggregate to use type-specific defaults rather than 'distinct count', mimcing the behavior of <perspective-viewer>