#read: for magma reader

[Chris Muller]

> Anybody see any problem with that #read: implementation
> to be able to count magma things?

Hi Sebastian, are you wanting to tally the number of dates for each
*unique* date, a la classic "control-break" reporting?

Or, are you simply wanting all elements in a MagmaCollection sorted by
date, in which case there is MagmaCollection>>#read:..?

But I see you want to extend #read: on MagmaCollectionReader, so I'm
not sure what that is for..