Fully featured DiseasyModelOde

[RasmusSkytte]

Intent

Fixes #154, #141

This PR improves the implementation of DiseasyModelOde and DiseasyModelOdeSeir

Approach

• The $get_results() function can now be called for DiseasyModelOde.
• Mapping from the model infections to different observables is now supported.
• A $plot() method has been added to easily visualise the model results (works with the plot() generic)
• Using different age stratification than the data source is now supported by DiseasyModelOde
• In initialise_state_vector we now actually use "incidence" for the input data (previously we used a different normalisation by accident and providing the incidence gave wrong results (The "SEIR-initialisation" vignette has been updated accordingly)
• A new vignette has been added to show case how to use DiseasyModelOdeSeir.

In addition, a number of fixes has been made and features added

• A flag is added to DiseasyObservables$get_observation called "respect_last_queryable_date" to allow the fetching of data
  beyond the last_queryable_date.
• data base connections in DiseasyObservables are now closed as expected.
• hash_environment has been improved to allow arbitrarily nested functions.
• Lots of additional tests has been added

Known issues

Waiting on the following

• Release SCDB 0.5.0 SCDB#167
• Release diseasystore 0.3.1 diseasystore#206
• New DiseasyImmunity module and waning function #101

Tests fail on r-devel. I can't wait to see how R manages to make dates "equal" but not "identical".

Checklist

• The PR passes all local unit tests
• I have documented any new features introduced
• If the PR adds a new feature, please add an entry in NEWS.md
• A reviewer is assigned to this PR

[kaare-gr]

Vignettes may be reviewed further separately.

[RasmusSkytte]

Vignettes updated after discussion with Kaare -- merging