Scott Kirsner writes: Two things are almost never free in the life of an urban motorist: parking and fuel. So when I started to hear buzz among electric vehicle drivers in Boston and Cambridge that they were enjoying both, I had to check it out. On a drizzly day last week, I hopped into a white BMW coupe — one of 700 all-electric vehicles the company has leased to US drivers — and started hunting for these mythical free parking spots and complimentary charging stations.