When the Women in Music Gathering Showcase gets going at the Burren in Somerville on Nov. 18, there will be 11 women onstage, each of them ready to perform a couple of songs, many of them ready to back up others on instruments or in voice. The event marks the first time this alliance of mostly singer-songwriters – which was formed last spring – will play out with this many of its 15 members.It’s the brainchild of singer-songwriter-body psychotherapist and Waltham [...]