A hand-drawn map leading to half a dozen artists’ studios was the tiny start of a festival that has become a huge institution in the Cambridge art scene. When two artists, Julia Ball and Christine Fox, decided in 1974 to throw open the doors of their workshops to help demystify the creative process and make art more accessible to ordinary people, they could not foresee the movement that would still be going half a century later. But Cambridge Open Studios is celebrating its 50th anniversary this year and it is still going strong – now with more than 500 members and more than 350 artists who ar…