Fix get_load_data bug using AbstractDeterministic

[GabrielKS]

The get_load_data function was manually filtering to only Deterministic time series, which was excluding DeterministicSingleTimeSeries. The fix is to use AbstractDeterministic to catch both. Here I do that and add a test.

See also NREL-Sienna/InfrastructureSystems.jl#378.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.16%. Comparing base (f976364) to head (68eee1b).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #31   +/-   ##
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  Coverage   88.16%   88.16%           
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  Files           3        3           
  Lines         321      321           
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  Hits          283      283           
  Misses         38       38           

Flag	Coverage Δ
unittests	88.16% <ø> (ø)

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