Move dotdict to dune-python (more precisely: to its python module dune.common)

I want to use ini files in a different context.

It would be nice to have an inifile parser in dune.common, that behaves exactly like its C++ counterpart.

So I intend to:

• move the DotDict class
• implement a pyparsing parser (or evaluate whether ConfigParser achieves the same thing easily).

@tkoch If you do not object, I will do this (soon).