Add reading for an external model

[acitrain]

In case we are using C++ executable, we need to be able to read an external model and not use the default value in the code.

This will be done in two parts:

-First by reading an external binary provided by the user (which as already been translated from SEP file or other)

• A second step will be to optimise the reading using ADIOS
• Then we can think about other efficient reading or maybe readers for specific format