Add support for partial model merge policy and default actions

[mfsoliveira]

For large languages or prototyping it is difficult to define and test merge policies for all concepts at the same time.
For this reason I extended the model merge language to support the modeling of a partial merge policy and the replication of default actions for concepts in without policies.

[mfsoliveira]

After some experiments trying to create default policies, we saw that we could not avoid the definition of merge policy at least for elements found the model, because the identity function must be defined.

As workaround I just reduce errors to warnings when finding missing policies. Additionally I added a flag to indicate an user is interesting to use a partial merge policy, so that some assertions and checks during the merge could be ignored.
Those changes are implemented in #1079