PR for release 0.3.0

[MartaVanin]

There are the following changes w.r.t. the last version:

• ADD/DOC: tests/examples and docs for state estimation with single-phase connections/networks and matpower data parsing, closes #41 , closes #13
• UPD: to PowerModelsDistribution v0.11.1 and InfrastructureModels 0.6.0 (breaking!)
• REF: adds core/export.jl with all the exported functions instead of keeping them in the _PMDSE module
• REF: removes PowerModels dep, all functions imported via PowerModelsDistribution. PowerModels still used in tests though (matpower parsing)
• REF: removes Memento dependency, logging is made consistent with PowerModelsDistribution v0.11 using Julia standard library
• DOC: edits readme

[codecov[bot]]

Codecov Report

Merging #42 (197e7c7) into master (472b728) will decrease coverage by 1.28%. The diff coverage is 62.16%.

@@            Coverage Diff             @@
##           master      #42      +/-   ##
==========================================
- Coverage   71.01%   69.72%   -1.29%     
==========================================
  Files          15       16       +1     
  Lines        1297     1341      +44     
==========================================
+ Hits          921      935      +14     
- Misses        376      406      +30     

Impacted Files	Coverage Δ
src/io/distributions.jl	76.54% <0.00%> (-7.41%)	:arrow_down:
src/form/reduced_ac.jl	19.35% <13.33%> (-2.08%)	:arrow_down:
src/prob/se.jl	66.15% <35.29%> (-1.54%)	:arrow_down:
src/form/reduced_ivr.jl	35.66% <46.15%> (-0.51%)	:arrow_down:
src/core/logging.jl	46.66% <46.66%> (ø)
src/core/measurement_conversion.jl	59.33% <52.63%> (+0.47%)	:arrow_up:
src/core/utils.jl	81.81% <76.47%> (+0.86%)	:arrow_up:
src/form/adapted_pmd_constraints.jl	74.31% <80.88%> (+0.54%)	:arrow_up:
src/core/variable.jl	95.23% <81.25%> (-4.77%)	:arrow_down:
src/PowerModelsDistributionStateEstimation.jl	100.00% <100.00%> (ø)
... and 10 more

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