add package path

It is now possible add the path of a Modelica package to the Simulator class.

The default is None. When is None the environmental variable MODELICAPATH is checked and used as path, if not available the current working directory is used. Then there is a check that controls if the selected directory contains a file called package.mo, if not an exception is raised.

I also added a check when the Simulator copy all the directory tree in the tmp folder.
We should control when copying the data (I had an issue with a broken symlink and this was preventing me to copy the package). For the unit test of the Buildings library it is important but perhaps it is not so important in other cases.

Documentation and unit test updated.