RelaxConstVolSetGenerator: ISIF 2 or ISIF 4

[JaGeo]

Shouldn't a Maker with constant volume use ISIF 4 rather than ISIF 2?

#1247 (comment)_

[esoteric-ephemera]

Both ISIF 2 and 4 preserve the cell volume, but the user may or may not want to preserve lattice shape no? Think it's best to leave that to the user's discretion