Remove t_start, rename reference_date->start_date

[Sbozzolo]

t_start and reference_date were added following an incorrect assumption (that restarts would restart at t = 0). The names were taken from ClimaLand, where these variables were used slightly differently (reference_date might have been the first date available in a file, and t_start allowed to run at at different point in that file).

Passing Atmos job: https://buildkite.com/clima/climaatmos-ci/builds/20465

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 75.00000% with 4 lines in your changes missing coverage. Please review.

Project coverage is 84.90%. Comparing base (d2feca0) to head (1eac484). Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/Schedules.jl	69.23%	4 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #75 +/- ## ========================================== - Coverage 85.48% 84.90% -0.59% ========================================== Files 13 13 Lines 503 510 +7 ========================================== + Hits 430 433 +3 - Misses 73 77 +4 ```

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