RangeCoverage errors on datetime columns in DiagnosticReport

[frances-h]

Resolve #431

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.11% :tada:

Comparison is base (6f17fc5) 76.34% compared to head (616d82a) 76.45%.

:exclamation: Your organization is not using the GitHub App Integration. As a result you may experience degraded service beginning May 15th. Please install the GitHub App Integration for your organization. Read more.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## diagnostic-report-properties #436 +/- ## ================================================================ + Coverage 76.34% 76.45% +0.11% ================================================================ Files 91 91 Lines 3301 3317 +16 ================================================================ + Hits 2520 2536 +16 Misses 781 781 ``` | [Files Changed](https://app.codecov.io/gh/sdv-dev/SDMetrics/pull/436?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None) | Coverage Δ | | |---|---|---| | [sdmetrics/reports/base\_report.py](https://app.codecov.io/gh/sdv-dev/SDMetrics/pull/436?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-c2RtZXRyaWNzL3JlcG9ydHMvYmFzZV9yZXBvcnQucHk=) | `98.93% <100.00%> (+0.15%)` | :arrow_up: | | [...ics/reports/multi\_table/base\_multi\_table\_report.py](https://app.codecov.io/gh/sdv-dev/SDMetrics/pull/436?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-c2RtZXRyaWNzL3JlcG9ydHMvbXVsdGlfdGFibGUvYmFzZV9tdWx0aV90YWJsZV9yZXBvcnQucHk=) | `97.72% <100.00%> (+0.22%)` | :arrow_up: |

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

[frances-h]

@fealho I realized that we probably want to convert datetimes across all reports, not just the DiagnosticReport so I moved it back into the BaseReport.