add nice explanatory output to extract-partitions

[ctb]

In particular,

• print out warning (& final count) when eliminating small partitions, if option set;
• allow -X to be a floating point number (i.e. apply int())

[mr-c]

Does this need to be in v0.8 or can it wait?

[ctb]

No version dependency is specified...? So I think whenever is fine, it's just for cleanup/enhancement.

[RamRS]

I could take this on, if it doesn't interfere with any other enhancement in progress!

[ctb]

Could be done along with #144, which also involves updating extract-partitions.