Allow a logical intervals table as input to select / remove intervals

Make this work:
```
sa_temps = fetch.Msid('TSAPYT','2000:001',stat='5min') 
roll = fetch.Msid('ROLL','2000:001',stat='5min')

off_nom = utils.logical_intervals(roll.times, abs(roll.vals) > 20)
sa_temps.select_intervals(off_nom)
```