Reduces visibility of some MortonContext methods.

Corrects a naming mistake (numberOfLeaves did not return the number of
leaves, but the square root of number of leaves).
Fixes bug for MortonCode

MortonCodes were incorrectly calculated for contexts where extent height is
different from width.
Reduces visibility of some MortonContext methods.

Corrects a naming mistake (numberOfLeaves did not return the number of
leaves, but the square root of number of leaves).
Fixes bug for MortonCode

MortonCodes were incorrectly calculated for contexts where extent height is
different from width.
Extends MortonCode interface with envelopeOf(mortoncode) method.
Merged with upstream.