Fixed setting the previous temperature value in the OGS MFront interface

This MR fixes only a small typo. However, that typo might affect all simulations of OGS with MFront behaviours that use temperature dependent MFront behaviours!

Before this MR the temperature T was always 0 (wrong) in the MFront behaviour and dT was current T - 0 (wrong). But T + dT = current T (right). So if your MFront behaviours use T or dT separately, you are affected, but if they use only T + dT you are probably lucky.

• Feature description was added to the changelog