Storage addition to transient model

[kaarthiksundar]

1. Completely refactored the transient OGF formulation to include storage components.
2. Additionally, the constraints and variables are modified according to suggestions from Eric to make the solve more faster with Ipopt.
3. Added storage unit tests.
4. Modified make_si_units! to perform conversion of all storage fields to SI.
5. Fixed documentation and CHANGELOG.

[codecov]

Codecov Report

Merging #217 into master will increase coverage by 1.78%.
The diff coverage is 77.74%.

@@            Coverage Diff             @@
##           master     #217      +/-   ##
==========================================
+ Coverage   76.47%   78.25%   +1.78%     
==========================================
  Files          35       35              
  Lines        3919     4489     +570     
==========================================
+ Hits         2997     3513     +516     
- Misses        922      976      +54     

Impacted Files	Coverage Δ
src/GasModels.jl	25.00% <ø> (ø)
src/core/types.jl	100.00% <ø> (ø)
src/core/unit_converters.jl	0.00% <0.00%> (ø)
src/io/diagnostics.jl	0.00% <0.00%> (ø)
src/form/lrwp.jl	50.33% <37.03%> (+1.68%)	⬆️
src/form/lrdwp.jl	43.47% <45.83%> (+2.17%)	⬆️
src/core/objective.jl	64.70% <54.54%> (+3.76%)	⬆️
src/form/wp.jl	70.66% <55.84%> (+2.55%)	⬆️
src/core/constraint_mi.jl	59.61% <58.64%> (+2.13%)	⬆️
src/io/matgas.jl	67.06% <62.16%> (+2.82%)	⬆️
... and 43 more

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