normalize_soil_pct: Normalize soil sand, silt, clay, and rock percentages to 100%
In jinshijian/UrbanKfs: UrbanKFS modeling project

If no Percent_Rock_Fragment is given (Percent_Rock_Fragment = NULL, default), renormalize sand, silt, and clay percents so that they sum to 100%. If rock is given, normalize sand, silt, and clay so that their sum = Percent_Rock_Fragment - 100.

normalize_soil_pct(
  Percent_Sand,
  Percent_Silt,
  Percent_Clay,
  Percent_Rock_Fragment = NULL
)

normalize_soil_pct_data(df, add_rock = TRUE)

`Percent_Sand, Percent_Silt, Percent_Clay`	Percentages (0 - 100%) of sand, silt, and clay in soil
`Percent_Rock_Fragment`	Percent rock in soil. If `NULL` (default), re-normalize sand, silt, and clay to 100%
`df`	`data.frame` containing soil percentages in columns `soil_cols`
`add_rock`	(Logical) If `TRUE`, add rock data