Minor, non-functional changes.
'NestedProperty' now implements 'ReadOnlyNestedProperty'.
Fixed tests, so their value comparison does not rely on interning.
Fixed some comments.
Implemented 'NestedDoubleProperty' including the builder and tests.
Property nesting builders now return the most specific types of properties, namely those which extend 'Simple...Property' and implement 'NestedProperty'.
Implemented 'NestedFloatProperty' including the builder and tests.
Fixed some comments.
Minor harmonization of some tests.
Implemented 'NestedLongProperty' including the builder and tests.