There aren’t many companies looking to double the size of their offices these days, particularly in downtown Boston. But that’s exactly what SimpliSafe did: The provider of alarms, cameras, and other home-security devices recently opened its new headquarters at 100 Summer St. Moving from nearby 294 Washington St., SimpliSafe nearly doubled its space to 150,000 square feet over floors three through six in the office tower, up from 81,000 square feet in the old location.