Add 6 Hour Snowfall Service

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (74065f1) 84.78% compared to head (55c0918) 85.03%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #158 +/- ## ========================================== + Coverage 84.78% 85.03% +0.25% ========================================== Files 64 66 +2 Lines 2057 2092 +35 ========================================== + Hits 1744 1779 +35 Misses 313 313 ``` | [Files](https://app.codecov.io/gh/akrherz/iem-web-services/pull/158?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=daryl+herzmann) | Coverage Δ | | |---|---|---| | [src/iemws/main.py](https://app.codecov.io/gh/akrherz/iem-web-services/pull/158?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=daryl+herzmann#diff-c3JjL2llbXdzL21haW4ucHk=) | `97.93% <100.00%> (+0.02%)` | :arrow_up: | | [src/iemws/models/nws/snowfall\_6hour.py](https://app.codecov.io/gh/akrherz/iem-web-services/pull/158?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=daryl+herzmann#diff-c3JjL2llbXdzL21vZGVscy9ud3Mvc25vd2ZhbGxfNmhvdXIucHk=) | `100.00% <100.00%> (ø)` | | | [src/iemws/services/isusm/daily.py](https://app.codecov.io/gh/akrherz/iem-web-services/pull/158?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=daryl+herzmann#diff-c3JjL2llbXdzL3NlcnZpY2VzL2lzdXNtL2RhaWx5LnB5) | `86.27% <ø> (ø)` | | | [src/iemws/services/nws/snowfall\_6hour.py](https://app.codecov.io/gh/akrherz/iem-web-services/pull/158?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=daryl+herzmann#diff-c3JjL2llbXdzL3NlcnZpY2VzL253cy9zbm93ZmFsbF82aG91ci5weQ==) | `100.00% <100.00%> (ø)` | | | [src/iemws/services/obhistory.py](https://app.codecov.io/gh/akrherz/iem-web-services/pull/158?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=daryl+herzmann#diff-c3JjL2llbXdzL3NlcnZpY2VzL29iaGlzdG9yeS5weQ==) | `68.75% <ø> (ø)` | |

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