Implement interaction with magnetic field in fronting medium

As it was worked out in #18 (closed), magnetic fiend in fronting medium is ignored in calculations. It concerns both external field and z-components of magnetization of all layers. Due to this reason and for not mislead users who will try to set these fields and to see what happens, these components are blocked for manual setting in GUI and Python script.

After resolving this issue, grep for '#654' in code and restore functionality.