add collatz conjecture exercise (#804)

Closes #803.

Author: kgengler

exercises/collatz-conjecture/canonical-data.json

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+{
+  "exercise": "collatz-conjecture",
+  "version": "1.0.0",
+  "cases": [
+    {
+      "description": "zero steps for one",
+      "property": "steps",
+      "number": 1,
+      "expected": 0
+    },
+    {
+      "description": "divide if even",
+      "property": "steps",
+      "number": 16,
+      "expected": 4
+    },
+    {
+      "description": "even and odd steps",
+      "property": "steps",
+      "number": 12,
+      "expected": 9
+    },
+    {
+      "description": "zero is an error",
+      "property": "steps",
+      "number": 0,
+      "expected": { "error": "Only positive numbers are allowed" }
+    },
+    {
+      "description": "negative value is an error ",
+      "property": "steps",
+      "number": -15,
+      "expected": { "error": "Only positive numbers are allowed" }
+    }
+  ]
+}

exercises/collatz-conjecture/description.md

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+The Collatz Conjecture or 3x+1 problem can be summarized as follows:
+
+Take any positive integer n. If n is even, divide n by 2 to get n / 2. If n is
+odd, multiply n by 3 and add 1 to get 3n + 1. Repeat the process indefinitely.
+The conjecture states that no matter which number you start with, you will
+always reach 1 eventually.
+
+Given a number n, return the number of steps required to reach 1.
+
+## Examples
+Starting with n = 12, the steps would be as follows:
+
+0. 12
+1. 6
+2. 3
+3. 10
+4. 5
+5. 16
+6. 8
+7. 4
+8. 2
+9. 1
+
+Resulting in 9 steps. So for input n = 12, the return value would be 9.
+

exercises/collatz-conjecture/metadata.yml

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+---
+blurb: "Calculate the number of steps to reach 1 using the Collatz conjecture"
+source: "An unsolved problem in mathematics named after mathematician Lothar Collatz"
+source_url: "https://en.wikipedia.org/wiki/3x_%2B_1_problem"