The number of townhouse sales fell 15.5% to 196 from last year to their lowest level since 2010. As a result, the market share of all residential sales, including co-ops and condos, fell to 1.9%, the lowest level reached since 2010. Townhouses are considered a luxury submarket and market conditions have been softening over the past several years, much like the luxury apartment market. Listing inventory slipped by 1.9% to…