DEV: Add sample_design class and re-implement design_effect

[fredjaya]

Part of #40

Adds two sample_design classes, fixed_design and variable_design to:

• reduce the repetition of input parameters across functions
• separate the logic of the sampling approaches
• drop the function naming _random (to come)

Changes the design_effect implementation:

• From functions to methods dispatched according to sample_design class - this removes the need for a separate function named with _random

Adds check_input2.R:

• See Rework check_input() #41
• Appended new functions with '2' to distinguish from check_input.R functions

Added and updated tests and documentation for sample_design and `design_effect

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.37%. Comparing base (7ab5a24) to head (0e43edf).

❗ Your organization needs to install the Codecov GitHub app to enable full functionality.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #42      +/-   ##
==========================================
+ Coverage   89.58%   90.37%   +0.78%     
==========================================
  Files          12       14       +2     
  Lines         989     1018      +29     
==========================================
+ Hits          886      920      +34     
+ Misses        103       98       -5     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.