First try to implement the flat hierarchy resolution strategy as described here #128

The following resolver-strategies are implemented:

• recursive-nested: this is default resolver and represents the well known basic gitman recursive nested resolution strategy
• flat: enforce gitman to ignore nested gitman.yml-files is similar as -d 1 (as described here Add recurse option in config #235).
• recursive-flat: resolves all nested depedencies in a flat manner as described in Flat hierarchy resolution #128 + Possible Feature Request - Nested Dependencies Organization #175
• recursive-flat-nested-links: is basically recursive-flat + creating nested symlinks which means that it looks like recursive-nested for the dependencies but the nested imported dependencies are created as symlinks which are linked to the toplevel location

This work is still in progress a couple of things need to be addressed.
Currently the basic commands update, lock and install work with all resolver but still some tests are broken. There are also some workarounds introduced to avoid that datafiles persist all fields. I hope there is a better solution out there but I didn't find them.

There are also some open question how gitman should deal if the resolver settings are different between the toplevel gitman file and the nested gitman files. Currently the resolver setting of the toplevel gitman file will always override the nested resolver settings.