Phlex/FORM integration for multiple input files

[knoepfel]

This discussion is continues discussion #505, which states that the following is required for sometime after Phlex v0.3.0:

1. The framework will need to be able to update the processed data-layer hierarchy when opening another input file.
2. The framework may support a running mode where all input files handled by a data-management system are declared to adhere to a specific hierarchy, thus avoiding unnecessary checking of the hierarchy contained in each file.
3. More than one input file may need to be open at the same time to support multi-threaded execution (transitions between input files).
4. More than one input file may need to be open at the same time to support (e.g.) data-MC overlay.