Make accessing the CddsPlugin nicer

Migrated Jira Issue CDDSO-167
Jira creation date 06/07/2022 10:35
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When the plugin is accessed, the instance of the ``PluginStore`` must be accessed and then the plugin:

```PluginStore.instance().get_plugin()```

Because now we only store one plugin instead of multiple, we could simplify it a bit by provide a ``classmethod`` ``property`` function:

```PluginStore.plugin```

This would simplify many calls and make the code more compact.
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Search for PluginStore and the existing PluginStore.instance().get_plugin() calls to identify the affected entry points. Replace the single-plugin access pattern with PluginStore.plugin and verify that all existing plugin access continues to work through the project's test suite.

Written by the indexing model from the issue text.