Avoid calls to the API on CI and CRAN

[KevCaz]

In this PR

1. Unit tests now use vcr to avoid direct calls to the web APIs (and avoid being archived repeatedly on CRAN):
   • cassettes are in tests/fixtures
   • add vcr in Suggests
2. The vignette has been precomputed (again to avoid calls to web API).
3. All examples are within the \donttest tag and the CI will ignore those examples (but one can easily run those locally).
4. Travis and Appveyor have been dropped and replaced by GitHub Actions.
5. get_collection() methods always return an object of class mgNetworksCollection (Principle of least astonishment), to that end get_network_by_id() gained an argument force_collection.

[codecov-io]

Codecov Report

Merging #100 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #100   +/-   ##
=======================================
  Coverage   96.26%   96.26%           
=======================================
  Files          12       12           
  Lines         375      375           
=======================================
  Hits          361      361           
  Misses         14       14           

Impacted Files	Coverage Δ
R/zzz.R	96.89% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 22bad46...df28382. Read the comment docs.

[KevCaz]

Ready to go.