mpi: Fix data distribution bugs [part 2]

[rhodrin]

Fix for issue #1862 + associated tests.

WIP. Will update.

[codecov[bot]]

Codecov Report

Merging #1949 (29112c5) into master (12995d3) will increase coverage by 0.00%. The diff coverage is 92.68%.

@@           Coverage Diff            @@
##           master    #1949    +/-   ##
========================================
  Coverage   87.90%   87.91%            
========================================
  Files         214      214            
  Lines       36504    36613   +109     
  Branches     5513     5538    +25     
========================================
+ Hits        32090    32189    +99     
- Misses       3897     3907    +10     
  Partials      517      517            

Impacted Files	Coverage Δ
devito/data/utils.py	89.12% <18.18%> (-3.46%)	:arrow_down:
devito/data/data.py	95.79% <100.00%> (+0.62%)	:arrow_up:
tests/conftest.py	84.41% <100.00%> (-0.11%)	:arrow_down:
tests/test_data.py	97.87% <100.00%> (+0.11%)	:arrow_up:
devito/ir/support/basic.py	91.98% <0.00%> (-0.22%)	:arrow_down:

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