Fix SingleTaskOperation on Argo worflows

Is my understanding that something like:

```
my_task = tasks.CustomTask('print-file', 'myapp-mytask')
op = operations.SingleTaskOperation('my-op-', my_task)
op.execute()
```
Would fail with the error:

```
[workspaces] File "/usr/src/app/api/models/population.py", line 59, in save
[workspaces] execute_generate_population_images_workflow(self.id)
[workspaces] File "/usr/src/app/api/services/workflows_service.py", line 17, in execute_generate_population_images_workflow
[workspaces] operations.SingleTaskOperation(
[workspaces] File "/libraries/cloudharness-common/cloudharness/workflows/operations.py", line 55, in execute
[workspaces] raise NotImplementedError(f"{self.__class__.__name__} is abstract")
[workspaces] NotImplementedError: SingleTaskOperation is abstract
```

CH Version: 1.0.0

Start with cloudharness/workflows/operations.py around line 55 and trace the SingleTaskOperation call from api/services/workflows_service.py. Reproduce the shown CustomTask and SingleTaskOperation example, then verify the intended Argo workflow behavior no longer raises the abstract-operation error.

Written by the indexing model from the issue text.