Preserve Units on New (NX)

#### Description:

When a new model is opened using the API call the units are automatically set to the MidasCivil default units and might be different from the original reference model.

#### Steps to reproduce:

#### Expected behaviour:

#### Test file(s):

No source file, entry point, reproduction steps, expected behavior, or test file is named. Start by locating the API call that opens a new model and trace how it selects units versus the original reference model; done means the original units are preserved and a regression test covers the behavior.

Written by the indexing model from the issue text.