Changes in layer dynamics and enthalpy calculations

• /mo_grotz.f90
• /mo_layer_dynamics.f90: made middle layer thickness more dynamic to improve handling of ice core initialization runs
• /mo_mass.f90: remove brine mass from total enthalpy in expulsion function
• /mo_data.f90: corrected confused enthalpy descriptions

requested review from @baz7650

assigned to @baz7650

mentioned in commit 0b59931e

merged