MainEnsightOutput: change the template parameter

When using it on third party libraries, it could be awkward at times because we must specify a specifi model where in fact we just want the underlying data type.

For instance for (custom) Hyperelasticity same Ensight output is used for CiarletGeymonat/StVenantKirchhoff and midpoint/half_sum, but I have to specify one of these models arbitrarily.