P and Q weights in C++ divergence and gradient superfluous and confusing

[jbrzensk]

Describe the bug
The P and Q weights in divergence.cpp and gradient.cpp and associated divergence.h and gradient.h are not technically correct and confuse users.

To Reproduce
View the source code

I suggest removing them, and implementing a similar function to weightsP.m and weightsQ.m.
It would be sufficient to just remove the references in the C++ code.

[joehellmersNOAA]

@jbrzensk I will work on adding the P and Q weights to C++. Should I do it against this ticket, or should I make a new one?

[jbrzensk]

@joehellmersNOAA yes, just use this one