Add surv Verb to Estimate a Survival Curve (#1788)

Add a surv verb to estimate a survival curve using Kaplan-Meier.  It
requires duration and status (event or censored) columns, and outputs
each distinct duration and corresponding probability of survival.

test/cases/cli-help/0001/expout

@@ -1261,6 +1261,16 @@ Options:
 --transpose        Show output with field names as column names..
 -h|--help Show this message.
 
+================================================================
+surv
+Usage: mlr surv -d {duration-field} -s {status-field}
+
+Estimate Kaplan-Meier survival curve (right-censored).
+Options:
+  -d {field}   Name of duration field (time-to-event or censoring).
+  -s {field}   Name of status field (0=censored, 1=event).
+  -h, --help   Show this message.
+
 ================================================================
 tac
 Usage: mlr tac [options]

test/cases/verb-surv/0001/cmd

@@ -0,0 +1 @@
+mlr --csv --from test/input/surv.csv surv -d duration -s status

test/cases/verb-surv/0001/expout

@@ -0,0 +1,4 @@
+time,survival
+1.00000000,0.80000000
+3.00000000,0.53333333
+5.00000000,0.00000000

test/input/surv.csv

@@ -0,0 +1,6 @@
+duration,status
+1,1
+2,0
+3,1
+4,0
+5,1