Better File handling during restart for G0W0

Previously, the files were created by the Contexts without any knowledge of whether previous files existed in the FileCache or not. This is because the FileCache was created in G0W0Calculator.init created at the end of G0W0.init via super. By doing this, restarting calculations always overwrites previous files.

In this MR, we move the FileCache to G0W0.init so we can ask the FileCache if files exists and properly decide whether to restart via append mode or write a new file. This means Contexts now must be able to accept append or write mode. Now we pass qcache to G0W0Calculator instead of constructing it in this object.

Closes #574 (closed)