Add ArrayDifferentialOperators for Vector calculus

[xtalax]

Adds Vector calculus operators which are composable by the dot and cross product.

A request for comment on a good design so that equations with these display intelligibly

[codecov-commenter]

Codecov Report

Merging #942 (068ad64) into master (60d26bb) will increase coverage by 0.44%. The diff coverage is 1.72%.

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@@            Coverage Diff            @@
##           master    #942      +/-   ##
=========================================
+ Coverage    8.35%   8.80%   +0.44%     
=========================================
  Files          26      26              
  Lines        3267    3318      +51     
=========================================
+ Hits          273     292      +19     
- Misses       2994    3026      +32     

Files Changed	Coverage Δ
src/diff.jl	0.29% <0.00%> (-0.06%)	:arrow_down:
src/arrays.jl	12.18% <100.00%> (+0.55%)	:arrow_up:

... and 1 file with indirect coverage changes

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[xtalax]

Core fails look unrelated

[shashi]

2 things:

1. There are extra Abstract types, I don't like creating type hierarchy until it leads to code reuse with dispatch. Until then, just use concrete types.
2. Ideally I'd want to give Differential itself dimensions. So Differential(x) behaves like d/dx for scalar, but Differential(x) behaves like Nabla for x::Vector. Can achieve nabla by just parameterizing Differential with the type of x. It seems like all the other operators you added can still be expressed in these terms.

[xtalax]

Ideally I'd want to give Differential itself dimensions. So Differential(x) behaves like d/dx for scalar, but Differential(x) behaves like Nabla for x::Vector. Can achieve nabla by just parameterizing Differential with the type of x. It seems like all the other operators you added can still be expressed in these terms.

The reason I have done this is that operators are often combined together, so the elements of differentials are not necessarily differentials, but can in general be other functions too.

[ChrisRackauckas]

What's left here?

[raphaelchinchilla]

What is the status of this PR?

[ChrisRackauckas]

Stale, someone would need to revive it.