add proj_epred

[avehtari]

closes #559

• adds proj_epred as a wrapper to proj_linpred, except that it doesn't return lpd, being more similar with proj_predict (I think it's enough that there is one function for returning lpd)
• improves documentation of proj_predict and proj_linpred, too

[fweber144]

Looks good. I just added two minor commits (b9c3563 and 4adcca6).