Update API, README and docs with minor fixes

[pitmonticone]

Minor bug fixes, code refactoring and docs update.

Closes #60.

[coveralls]

Pull Request Test Coverage Report for Build 3861784350

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 64 of 72 (88.89%) changed or added relevant lines in 6 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.008%) to 83.325%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/abstractmultilayergraph.jl	4	5	80.0%
src/multilayerdigraph.jl	27	29	93.1%
src/multilayergraph.jl	30	35	85.71%
<!--	Total:	64	72	88.89%	-->

Totals
Change from base Build 3830325291:	-0.008%
Covered Lines:	1679
Relevant Lines:	2015

---

💛 - Coveralls

[codecov[bot]]

Codecov Report

Merging #64 (f15f1a3) into main (458deea) will decrease coverage by 0.01%. The diff coverage is 88.88%.

@@            Coverage Diff             @@
##             main      #64      +/-   ##
==========================================
- Coverage   83.34%   83.32%   -0.02%     
==========================================
  Files          30       30              
  Lines        2011     2015       +4     
==========================================
+ Hits         1676     1679       +3     
- Misses        335      336       +1     

Impacted Files	Coverage Δ
src/multilayeredge.jl	53.33% <ø> (ø)
src/abstractmultilayergraph.jl	87.65% <80.00%> (-0.10%)	:arrow_down:
src/multilayergraph.jl	89.01% <85.71%> (ø)
src/multilayerdigraph.jl	90.69% <93.10%> (ø)
src/abstractmultilayerdigraph.jl	98.79% <100.00%> (ø)
src/abstractmultilayerugraph.jl	99.37% <100.00%> (ø)
src/subgraphs/layer.jl	75.81% <100.00%> (-0.09%)	:arrow_down:

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.