Rename and move adapter.py

[MakisH]

The main adapter file is currently cht/adapter.py. We should:

• Rename the adapter to something that points to preCICE, for example precice.py.
• Move it to the main directory, unless we have any good reason not to. An important design decision would be if we:
  1. implement FSI in the same file as CHT (preferably, if simple)
  2. split CHT and FSI in a similar way as in the OpenFOAM adapter (preferably, if complex)
  3. provide completely different adapters for CHT and FSI
• Update the documentation respectively.

@uekerman what do you think about the name? Is there a good reason to keep this directory structure?