BUG: wrong forward daily returns
BUG: wrong forward daily returns

current implementation calculates daily returns as forward returns divided
by number of days:
daily_returns = fwd_ret / days

This patch calculate the daily returns as the value the returns would have
had every single day for 'forward' days if they had grown at a steady rate
that is:
daily_returns = (fwd_ret+1)**(1./days) - 1

luca-s/alphalens,quantopian/alphalens,luca-s/alphalens,quantopian/alphalens