ENH: added a general framework for normalization and setting targets

for all objective functions.

- The normalization is useful for practically all update methods dealing
with constraints (e.g. SQP, MMA). The normalization factor can be either
given explicitly or, if not given, will be the value of the objective
function in the first optimisation cycle.
- The target value is useful when using the objective as a constraint in
constrained optimisation problems (e.g. drag - dragTarget). It should
only be used with update methods that understand the value of the
constraint (e.g. SQP, MMA) but not when the objective in hand is the
only objective of the optimisation problem. In such a case, a squared
objective should be used (e.g. sqr(drag - dragTarget))