cleanup in constructors, TreeMap, HashTable
 - constructors now maximize use of Foo(): a(), b() {} syntax
 - added parameter to TreeMap::_nodeFor
 - added a moveBackTo method to DoubleList to speed up HTable growth
Updated .gitignore, changed name of scons-built exe
Removed default implementations for Set, Map; must choose explicitly

To use trees, use Set<a, b>::T or Map<a, b>::T; similar for maps
Added a diagnose function to trees; calculates avg paths & density
cambio pequeño