0.6.9: Merge pull request #1039 from MattHJensen/expand_ctc

Contributor	PR#	Description
Cody Kallen	#981	identify and fix bug in TaxGains function (see #977 and #979)
Cody Kallen	#987	add calculation of MTR wrt e26270, partnership & S-corp income
Matt Jensen	#988	identify and fix bug in multiyear_diagnostic_table utility function
Martin Holmer	#1000	fix AMT bug that ignored value of AMT_CG_rt1 parameter
Martin Holmer	#1001	fix several other minor AMT CG bugs
Sean Wang	#948	add utility function that plots marginal tax rates by percentile
Anderson Frailey	#999	add ability to simulate Trump-style dependent care credit
Martin Holmer	#1012	add ability to simulate Clinton-style NIIT reform
Martin Holmer	#1021	remove self-employment tax from income tax total
Martin Holmer	#1032	add half of self-employment tax to expanded income
Matt Jensen	#1039	add ability to simulate Clinton-style CTC expansion