Refactoring and Sub-branches

Some refactoring: tree package now only contains information about
location, direction, geometry etc. and the app package deals with
actually creating 3-dimensional shapes from the data in the tree
package.
Some support of sub-branches added but not on a recursive level - only
direct sons can be added to every BranchShape object at the moment.
trees_data.txt file is added as a promo for things to come soon...