resolves some of the suggested improvements, renames the term period with cycle

lib/cycle-length.js

@@ -1,38 +1,38 @@
 import assert from 'assert'
 
-export default function getPeriodLengthStats(cycleLengths) {
+export default function getCycleLengthStats(cycleLengths) {
   throwIfArgsAreNotInRequiredFormat(cycleLengths)
-  const periodLengthStats = {}
+  const cycleLengthStats = {}
   const sortedCycleLengths = cycleLengths.sort((a, b) => {
     return a - b
   })
-  periodLengthStats.minimum = sortedCycleLengths[0]
-  periodLengthStats.maximum = sortedCycleLengths[cycleLengths.length - 1]
-  periodLengthStats.mean = Math.round(
+  cycleLengthStats.minimum = sortedCycleLengths[0]
+  cycleLengthStats.maximum = sortedCycleLengths[cycleLengths.length - 1]
+  cycleLengthStats.mean = Math.round(
     cycleLengths.reduce(getSum) / cycleLengths.length * 100
   ) / 100
   // median
   if (cycleLengths.length % 2 == 1) {
-    periodLengthStats.median = sortedCycleLengths[
+    cycleLengthStats.median = sortedCycleLengths[
       (cycleLengths.length + 1) / 2 - 1
     ]
   } else {
     const middle = cycleLengths.length / 2
-    periodLengthStats.median = (sortedCycleLengths[middle - 1] +
+    cycleLengthStats.median = (sortedCycleLengths[middle - 1] +
        sortedCycleLengths[middle]) / 2
   }
   // corrected standard deviation (based on unbiased sample variance)
   if (cycleLengths.length > 1) {
     const sumOfSquares = cycleLengths.map(cycleLength => {
-      return Math.pow(cycleLength - periodLengthStats.mean, 2)
+      return Math.pow(cycleLength - cycleLengthStats.mean, 2)
     }).reduce(getSum)
-    periodLengthStats.stdDeviation = Math.round(
+    cycleLengthStats.stdDeviation = Math.round(
       Math.sqrt(sumOfSquares / (cycleLengths.length - 1 )) * 100
     ) / 100
   } else {
-    periodLengthStats.stdDeviation = null
+    cycleLengthStats.stdDeviation = null
   }
-  return periodLengthStats
+  return cycleLengthStats
 }
 
 function getSum(total, num) {