DRY up the code to determine how much memory is available, still

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
WIP: Use MemoryTools *everywhere*

There is a completely arbitrary fraction in the plane separator now: we
ask wether a 16th of the available memory is enough to read the plane.
Probably this needs a little discussion...

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>