Add LoadXML function to WeberPennTree plugin

[jajberni]

This PR adds a function to load custom tree definitions from XML files, as currently available in the core Helios.

from pyhelios import Context, WeberPennTree
context = Context()
wpt = WeberPennTree(context)

wpt.loadXML('/tmp/custom-tree.xml')

new_tree_id = wpt.buildTreeFromName("CustomTree")

Unit tests have been added to add coverage to the new functionality.

Note that an absolute path is required for the XML. I had to implement the new function buildTreeFromName because buildTree only accepts WPTType. It would be nicer to generalise the tree names and add new tree definitions to the hard-coded trees, so that loading an XML file adds new names. A function like getAvailableTreeTypes could provide all the available names, but it would require adding a new public method into WeberPennTree.h and WeberPennTree.cpp.

E.g,:

std::vector<std::string> WeberPennTree::getAvailableTreeTypes() {
    std::vector<std::string> treeNames;
    for (const auto& tree : trees_library) {
        treeNames.push_back(tree.first);
    }
    return treeNames;
}

Then it would be possible to expose this function in Python and avoid hardcoding WPTType.