provide better interface for users of the package

``` R
qconfig = read(file="DO_sensor.yaml")
sensor = read(file="sensor.txt", format)
sensor = window(sensor, ...)
sensor = flag(sensor, qconfig) # works for windowed and non-windowed
 # -- or --
sensor = flag(sensor, ...) # for manual operations that are in sqc

qconfig['field'] = 'n=9'
write(qconfig, file, ...)

data = clean(sensor, flag.logic)
write(data, file, with.flags, ...)
```
