Refactoring of preferences management

* Reverted to openfluid.conf name for configuration file
* Moved PreferencesManager to openfluid::base namespace

(closes OpenFLUID/openfluid#406)
(closes OpenFLUID/openfluid#407)
Reorganization of the user data directory ($HOME/.openfluid on Unix)

* Introduced wares directory for simulators, observers and
  builder-extension binaries
* Introduced workspace directory for projects and wares development  
* Updated PreferencesManager according to the new organization
* Cleaned CMake config files

(closes OpenFLUID/openfluid#408)