Add issue template for comparison error

issue.md

+There was an issue with the following unit conversion:
+
+- Left unit: {}
+- Right unit: {}
+
+### Result/Expectation: 
+
+| Calculation | Result | Expectation | Did match |
+|-------------|--------|-------------|-----------|
+| Base unit   | {}     |             | []        |
+| factor * left = right | {} |       | [] |
+| factor * right = left | {} |       | [] |
+| factor * left = base | {} |        | [] |
+| factor * base = left | {} |        | [] |
+| factor * right = base | {} |       | [] |
+| factor * base = right | {} |       | [] |
+
+If there was an error with the calculation, please fill out the table above. Feel free to also add additional info here: 
+
+*Free text comment*
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