In an attempt to stay affordable and competitive, two small liberal arts colleges in Newton are considering merging, the schools announced Saturday.  The presidents of Mount Ida College (MA) and Lasell College (MA) said they hope that combining forces will help keep tuition as low as possible while maintaining the quality of both schools’ academics.  The announcement comes as small colleges across the country struggle to stay viable in an era of declining enrollment when families increasingly struggle to afford private college tuition.