Add examination/methods.md and related files

[martinholmer]

This pull request adds the examination/methods.md document as well as several files in the examination/taxcalculator directory that are used to conduct the examination of the Phase 1 dataset.

[donboyd5]

Looks great @martinholmer ! Don't yet know how to do reviews properly so I'll put minor comments here:

tax_microdata_benchmarking/examination/methods.md:

• line 50 "an" should be "a"
• lines 50-54 simply a note - in phase 2 we likely will have a 3rd file based on puf with methods similar to taxdata, that eventually may supplant the policy engine file. not sure if any foreshadowing is needed here but just want to mention it
• I am probably misunderstanding something - on line 17 we say we compare ACA-PTC and in 78-79 we say there's no logic to do so. Is this something we would do later on?

Other files:

• I am confused about clp, which has a dvars file and an empty json file but I do not see reference to a clp tax expenditure. I may just be missing something.

[martinholmer]

@donboyd5, Thanks for your helpful review. I've made several changes in response to your comments.