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