Remove generic parameter from Model

[andrenarchy]

By using the this type we can get rid of the generic Model parameter, possibly also allowing nice mixins (see #34). Defining models is now as elegant as it can be. 😛

Caveat

We lose the type safety on the data parameter in model constructors if the derived class does not implement a constructor à la

class Person extends Model {
  constructor(data?: Partial<Person>) {
    super(data);
  }
}

Issue #37 keeps track of this.

[codecov]

Codecov Report

Merging #36 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #36   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          19     19           
  Lines         472    472           
  Branches      129    129           
=====================================
  Hits          472    472

Impacted Files	Coverage Δ
packages/octonom/lib/collection.ts	100% <100%> (ø)	⬆️
packages/octonom/lib/validate.ts	100% <100%> (ø)	⬆️
.../octonom/test/data/models/group-with-references.ts	100% <100%> (ø)	⬆️
packages/octonom/test/data/models/person.ts	100% <100%> (ø)	⬆️
...ackages/octonom/test/data/models/person-account.ts	100% <100%> (ø)	⬆️
packages/octonom/test/data/models/cat.ts	100% <100%> (ø)	⬆️
packages/octonom/lib/model-array.ts	100% <100%> (ø)	⬆️
packages/octonom/lib/model.ts	100% <100%> (ø)	⬆️
packages/octonom/lib/errors.ts	100% <100%> (ø)	⬆️
packages/octonom/test/data/models/discussion.ts	100% <100%> (ø)	⬆️
... and 3 more

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