FMU creation from Feel++ code

[lberti]

We want to include Feel++-based models in larger workflows and interface them with models coming from other sources.
A possible choice is using Functional Mockup Units (FMUs), as they provide a standardized interface for communications and interactions among models having different origins.

Since it is possible to build FMUs from C code, following the examples in Reference-FMUs, and since Feel++ is a C++-based library, it is possible to construct FMUs from Feel++ code by exposing its models in the C programming language.

The Reference-FMUs repository is part of the Modelica organization and it is an active repository. Its configuration is based on CMake and it aims to test and implement the FMI standard. It could be included as a submodule of the feelpp repository to enable the development of FMUs from Feel++ code.