Add flexible temperature limits

[tblanke]

It would be nice to have the possibilty to change the min and max temperature from fix values to some which are different for every month.

[wouterpeere]

This can indeed be a nice addition. It is however only possible for L3 and L4 sizing, since the L2 sizing implicitely assumes constant temperature bounds

[wouterpeere]

Will be implemented in issue #45

[wouterpeere]

Almost done.
We need to check if the methods of optimizing load profile also work with flexible temperature limits.