[Development] Add the possibility to project the level-set to obtain a desired volume

[sblauth]

To incorporate volume constraints in topology optimization, the level-set can be projected (shifted by a constant) to achieve the desired volume. It would be nice to implement this in cashocs, so that users can specify volume equality and inequality constraints (equal inequality bounds should correspond to a equality constraint, or using only one variable) which can be achieved by shifting the level set function (before update_levelset is called)