Rework models

[coldfix]

• assign variable values via model

DONE:

• serialization format for session [27c35d8]
• allow to define param dicts in individual files (*.twiss.yml, *.beam.yml). This makes it a lot easier to add additional settings by just saving a file to the folder. [fbc3dc2]
• move .yml file into data folder [9357353]
• one model = one configuration = defaults + init files
• simplify OpenModel dialog: a simple open dialog should be sufficient [d96dcf5]
• models don't need a name (they just need to be addressed, and thats preferrably by their path) [718ef06]
• POD models [2ffda5b]