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 #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 :white_check_mark:

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

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Additional details and impacted files

```diff @@ 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 ```

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