Implement public interface for ufuncs

[rsokl]

mg.add, mg.sin, etc. are all implemented as mygrad-ufuncs. These are all registered subclasses of an un-inheritable abstract base class mg.ufunc.

Fixes a bug where mem-lock was not behaving correctly in conjunction with view targets, and when a boolean mask is specified.

Also adds matplotlib plots in some of the examples sections of various functions