Time interval cqp whole days

[arildm]

Fixes #379

[arildm]

@janiemi Could you check if this fixes #379? I couldn't resist changing a few other details in the logic, I hope you don't mind.

For instance, I didn't see the point of the second condition in

[int(_.text_datefrom) >= 20020101 & int(_.text_datefrom) <= 20031231 & int(_.text_dateto) <= 20031231]

so that is now simplified further to

[int(_.text_datefrom) >= 20020101 & int(_.text_dateto) <= 20031231]

We can assume that the datefrom of a text is always less than or equal to its dateto, right?

[janiemi]

@arildm, you are of course right: datefrom should always be less than or equal to dateto; if not, I’d consider it an error in the data. As far as I can see, the results would seem to be what I’d expect. Thanks!