Add the store account function

[antalszava]

Context: The features of the Configuration class are being replaced by functions in configuration.py. Saving a configuration to a configuration file is yet to be added.

Description of the Change: Adds the store_account user-convenience function that uses the save_config_to_file auxiliary function.

Changes the configuration.rst documentation page to mirror the changes and provide simple examples.

Benefits: Provides a simple way from with Python to configure the API related details in Strawberry Fields.

Possible Drawbacks: N/A

Related GitHub Issues: N/A

[codecov[bot]]

Codecov Report

:exclamation: No coverage uploaded for pull request base (demo@fdfc8db). Click here to learn what that means. The diff coverage is 100%.

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##             demo     #306   +/-   ##
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  Files           ?       47           
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  Hits            ?     6024           
  Misses          ?      195           
  Partials        ?        0

Impacted Files	Coverage Δ
strawberryfields/configuration.py	100% <100%> (ø)

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[antalszava]

@co9olguy thank you so much for your comments, I've reworded the description where necessary and applied the changes!

[antalszava]

Updated the texts where applicable as best as I could, reverted the test data and included examples in the docstring. Also caught a bug for the case when the directory returned by user_config_dir did not exist, added logic and tests covering this.

Let me know if you have further suggestions! :blush: