Rename Storage -> (Storage) Unit

[guimarqu]

task for #330

[codecov]

Codecov Report

Merging #471 (f154343) into master (f1941be) will not change coverage.
The diff coverage is 82.39%.

@@           Coverage Diff           @@
##           master     #471   +/-   ##
=======================================
  Coverage   82.28%   82.28%           
=======================================
  Files          51       51           
  Lines        4702     4702           
=======================================
  Hits         3869     3869           
  Misses        833      833           

Impacted Files	Coverage Δ
src/Algorithm/Algorithm.jl	100.00% <ø> (ø)
src/Algorithm/branching/branchingrule.jl	20.00% <ø> (ø)
src/Algorithm/divide.jl	54.54% <ø> (ø)
src/Algorithm/node.jl	77.77% <ø> (ø)
src/MathProg/types.jl	79.16% <ø> (ø)
src/Algorithm/storage.jl	76.86% <42.85%> (ø)
src/Algorithm/preprocessing.jl	76.75% <72.58%> (ø)
src/Algorithm/data.jl	75.00% <72.72%> (ø)
src/Algorithm/formstorages.jl	52.60% <75.00%> (ø)
src/Algorithm/colgenstabilization.jl	99.22% <97.61%> (ø)
... and 11 more

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[guimarqu]

@rrsadykov ready for review.

[rrsadykov]

StorageUnitContainer or UnitContainer

[rrsadykov]

Actually, any model should contain a storage (a dictionary of storage units).

Is there an advantage of having AbstractFormulation, could we derive Formulation and Reformulation directly from AbstractModel? I have an impression that everything defined for AbstractFormulation should be defined for AbstractModel and moved to ColunaBase

[guimarqu]

I changed comments related to the title of the PR only. I do each task of #330 in a separate PR because otherwise I'll get lost in my changes haha