Change memory usage of dual attack

[ludopulles]

Resolves #88

• The parameter use_lll in lwe_dual.py was unused.
• The short_vectors methods in reduction.py now consistently return a 4-tuple, containing sieving dimension.
• The memory cost of dual attack considers memory for storing the dual vectors, AND the FFT table.
• The runtime cost also takes into account building the initial FFT table, based on all the enumeration targets.
• Various typo fixes.
• Added note that the number of dual vectors required is assuming the Independence Heuristic.