Document more extensively

[jaspercooper]

Add MIDA and description of each step in:

• [ ] binary_iv_designer.R
• [ ] block_cluster_two_arm_designer.R
• [ ] cluster_sampling_designer.R
• [ ] DesignLibrary.R
• [ ] factorial_designer.R
• [ ] helpers.R
• [ ] mediation_analysis_designer.R
• [ ] multi_arm_designer.R
• [ ] pretest_posttest_designer.R
• [ ] process_tracing_designer.R
• [ ] randomized_response_designer.R
• [ ] regression_discontinuity_designer.R
• [ ] spillover_designer.R
• [ ] two_arm_attrition_designer.R
• [ ] two_arm_designer.R
• [ ] two_by_two_designer.R