Refactor backends

Author: ph-kev

docs/literate_example.jl

@@ -167,7 +167,7 @@ observations .= process_member_data(SimDir(simulation.output_dir))
 # The simplest backend is the `JuliaBackend`, which runs all ensemble members sequentially and does not require `Distributed.jl`.
 # For more information, see the [`Backends`](https://clima.github.io/ClimaCalibrate.jl/dev/backends/) page.
 eki = CAL.calibrate(
-    CAL.WorkerBackend,
+    CAL.WorkerBackend(),
     ensemble_size,
     n_iterations,
     observations,

docs/src/api.md

@@ -25,9 +25,11 @@ ClimaCalibrate.foreach_remotecall_wait
 ```@docs
 ClimaCalibrate.calibrate
 ClimaCalibrate.JuliaBackend
+ClimaCalibrate.HPCBackend
 ClimaCalibrate.DerechoBackend
 ClimaCalibrate.CaltechHPCBackend
 ClimaCalibrate.ClimaGPUBackend
+ClimaCalibrate.GCPBackend
 ClimaCalibrate.get_backend
 ClimaCalibrate.model_run
 ClimaCalibrate.module_load_string