Add support for broadcasting (Global)PencilArrays

[jipolanco]

Things to do:

• [x] make sure broadcasting works correctly (and efficiently) with permutations on
• [x] does it make sense to combine permuted and non-permuted arrays?
• [x] check that in-place broadcasting works correctly and doesn't allocate

Closes #3

[codecov-commenter]

Codecov Report

Merging #6 into master will increase coverage by 0.08%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master       #6      +/-   ##
==========================================
+ Coverage   95.63%   95.71%   +0.08%     
==========================================
  Files          15       16       +1     
  Lines         687      700      +13     
==========================================
+ Hits          657      670      +13     
  Misses         30       30              

Impacted Files	Coverage Δ
src/PencilArrays.jl	100.00% <ø> (ø)
src/broadcast.jl	100.00% <100.00%> (ø)

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