Adding operator+[=] and operator/[=] for FloatingArray and IntegerArray.

[joshuastorck]

• [ ] closes #xxxx
• [ ] tests added / passed
• [ ] passes git diff upstream/master | flake8 --diff
• [ ] whatsnew entry

This includes a design change that obviates the need for an ArrayView. Instead, every array has an internal offset. Shallow copy is achieved by copy constructor, though the current set of copy constructors don't yet support a slice. Deep copy is still achieved through the Copy virtual function.

More detailed explanation of the changes:

• Adding copy/move constructors for {Floating,Integer,Numeric}Array
• Adding various method for marking/getting nulls (valid bits) in integer arrays
• Changing data() and mutable_data() in NumericArray so that they return a pointer that starts at the array's offset
• Addition of Addable/Divisable classes (similar to Boost operators) for easy support of operator[+/]
• Unit test scaffolding for testing permutations of left/right hand side types on arithmetic operators
• Implementing IntegerArray::operator/, IntegerArray::operator+= FloatingArray::operator/=, FloatingArray::operator+=

[wesm]

There's a bunch of bits here that can be extracted and merged as useful utilities. I want to wait on pulling in operator implementations until the operator interface has been thought through some more: https://docs.google.com/document/d/1YmsV48iO6YNSxCIC84Xig5z-i9g4g7rzYTmCgUAxKRk/edit#heading=h.pzfqkdqgusm7

I'd like to take a crack at implementing some operators in a way that can evaluated either in single- or multi-threaded mode (without excessive code duplication -- so the code that runs things in parallel won't be too knowledgeable about the details of the operator except how invoke their virtual functions

[wesm]

I cherry-picked some of this as https://github.com/pandas-dev/pandas2/commit/a7f2a82a88d57e373419f796dc9a66430cc400ed

I'm going to do a little prototyping on operator evaluation generally, so will return to this hopefully soon