Input created via
```sh
bash -c 'for i in {1..10000} ; do echo "id$i;0.0" ; done' >./src/test/resources/samples/measurements-10000-unique-keys.txt
```
and output via baseline implementation.
Keys are short and very similar which improves chances for collision
and hence are good for testing.
Fixes#91
Adds test samples that can be used for unit tests or to verify
implementations via:
```bash
for sample in $(ls src/test/resources/samples/*.txt)
do
echo "Validating $sample"
rm -f measurements.txt
ln -s $sample measurements.txt
diff <(./calculate_average.sh) ${sample%.txt}.out
done
rm measurements.txt
```
For #61
