Sort arrivals in headway regularity indicator

Partially addresses #380 

Sorts the arrivals in the headway regularity indicator; without this,
the stop arrivals appear to be ordered arbitrarily, leading to problems
such as negative average headways.

Before fix, expected and observed headways for SEPTA rail from the failing test (in seconds):
```
(763.8461538461538,804.5877192982456)
(973.3684210526316,1226.2222222222222)
(823.7647058823529,952.0631578947368)
(620.6795580110497,786.873076923077)
(261.6949152542373,243.04929577464787)
(1254.6666666666667,1295.0576923076924)
(639.6610169491526,716.0268456375838)
(261.68674698795184,229.2604501607717)
(895.5,1049.12)
(419.54887218045116,378.3831168831169)
(141.42857142857142,141.264)
(-205.13513513513513,-174.9502487562189) <-- Negative average headway
(26.81564245810056,85.16190476190476)
```

After fix:

```
(1119.2307692307693,1135.2368421052631)
(1417.6842105263158,1395.138888888889)
(945.8823529411765,1007.2105263157895)
(1143.7734806629835,1242.6346153846155)
(1119.8305084745762,1152.6267605633802)
(1294.6666666666667,1311.8076923076924)
(967.1186440677966,1002.3355704697987)
(901.3775100401606,872.2154340836013)
(1290.9,1240.0)
(1241.954887218045,1305.8116883116884)
(1414.2857142857142,1461.808)
(1258.6756756756756,1215.676616915423)
(979.4245810055866,996.7523809523809)
```

Updates test to match new indicator output.