[inequality] Review of lecture and incorporate updates

lectures/_static/lecture_specific/inequality/usa-gini-nwealth-tincome-lincome.csv

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+year,n_wealth,t_income,l_income
+1950,0.8257332034366358,0.4424865413945875,0.5342948198773428
+1953,0.8059487586599331,0.42645440609359453,0.5158978980963707
+1956,0.8121790488050629,0.4442694287339922,0.5349293526208139
+1959,0.7952068741637917,0.43749348077061556,0.5213985948309421
+1962,0.8086945076579354,0.4435843103853641,0.5345127915054346
+1965,0.790414922568793,0.4376371546666345,0.7487860020887755
+1968,0.7982885066993517,0.42086207944388987,0.524239642738153
+1971,0.7911574835420266,0.4233344246090252,0.5576454812313467
+1977,0.7571418922185217,0.46187678800902604,0.5704448110072053
+1983,0.7494335400643017,0.4393456184644693,0.5662220844385908
+1989,0.7715705301674308,0.5115249581654219,0.6013995687471431
+1992,0.7508126614055308,0.47406506720767694,0.5983592657979548
+1995,0.756949238811026,0.48965523558400526,0.5969779516716914
+1998,0.7603291991801175,0.4911744158516885,0.5774462841723366
+2001,0.7816118750507022,0.523909299468113,0.6042739644967348
+2004,0.770035546952236,0.48843503839032615,0.5981432201792747
+2007,0.7821413776486992,0.5197156312086196,0.6263452195753294
+2010,0.8250825295193438,0.51959721201456,0.6453653328291932
+2013,0.8227698931835281,0.5314001749843356,0.6498682917772638
+2016,0.8342975903562243,0.5541400068900844,0.6706846793375284