datamash: add page (#2085)

pages/linux/datamash.md

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+# datamash
+
+> Tool to perform basic numeric, textual and statistical operations on input textual data files.
+
+- Get max, min, mean and median of a single column of numbers:
+
+`seq 3 | datamash max 1 min 1 mean 1 median 1`
+
+- Get the mean of a single column of float numbers (floats must use "," and not "."):
+
+`echo -e '1.0\n2.5\n3.1\n4.3\n5.6\n5.7' | tr '.' ',' | datamash mean 1`
+
+- Get the mean of a single column of numbers with a given decimal precision:
+
+`echo -e '1\n2\n3\n4\n5\n5' | datamash -R {{number_of_decimals_wanted}} mean 1`
+
+- Get the mean of a single column of numbers ignoring "Na" and "NaN" (literal) strings:
+
+`echo -e '1\n2\nNa\n3\nNaN' | datamash --narm mean 1`