Design of helper functions

[pratikunterwegs]

This issue is intended as a design discussion for helper functions in {scenarios}.

These functions are intended to be used within comparisons objects, but could also be made available to users for more ad hoc use. 

• are_comparable(): Function that takes two or more scenario class objects as input and checks which pair-wise comparisons are possible. This depends on whether the same function was called, and whether the parameter list has the same elements (throw warnings if the parameter list is identical).
• Should check whether some user specified parameters are identical among scenarios (e.g. if comparing different R0 in the same population, do not allow comparison where demography is different; such as when demographic groups have different age limits).  Could be used for only two objects, in which case it should return a single boolean; otherwise, a boolean matrix of whether any pair of scenarios are comparable. 
• select_comparable(): Function that takes two or more scenario class objects as input, as well as an optional string or integer specifying which should be considered the ‘baseline’ object. Runs are_comparable() and returns only those scenarios which are comparable as a list; the ‘baseline’ object is returned as the first element of the list. If no ‘baseline’ is specified, the first object is assumed to be the baseline with a warning or message to the user. 
• make_scenario_names(): Function to make informative names for scenario objects, based on their parameter combinations, and/or other user-specified name components. 
• read_scenario() / write_scenario(): Functions to read in and save a scenario specification from or to a file (structured Excel, JSON, YAML). 
• More helper functions as required.