on-the-fly extend strategies

[malb]

We should not bomb out when a user tries to use a dimension larger than what's in the strategy file but rather compute a strategy on the fly for large dimensions.

[joerowell]

Does this not assume access to the strategizer in fplll? I don't think fplll ships with those bits (i.e set_mdc.py is probably necessary, although I guess we could fix 64 cycles per node?)

[malb]

Nope, we'd guess the right block size/cost by extrapolation and just call the pruner