Refactoring the starship CLI and adding unit tests

Context: The command line interface (CLI), starship has a few options which can be used. However, its functionality can be extended and tests need to be included such that most caveats for bugs are covered.

In terms of portability, the idea is to keep the command line interface platform independent.

Description of the Change: Created a fixed CLI structure where there are two main types of options:

General options: specified with the -- prefix, e.g. --ping
Commands: specified by the name of the command e.g. run

In more details this looks as follows: There are two general options: --help and --ping, in more details:

starship: returns the help menu
starship --help: returns the help menu
starship --ping: tests the connection to the remote backend

There are two commands: configure and run, in more details:

Configure

starship configure [--local]: interactive wizard
starship configure [--local] --token: only the authentication token with default configurations
(in both cases the --local option creates the configuration file locally)

Run

starship run bbscript.xbb: run a blackbird script
starship run bbscript.xbb --ouput myhome/mydirectory/myresult.txt: run a blackbird script and store the result

In terms of code structure, the ability to create unit tests was regarded (as an example, suggestions on how to write tests for the argparse method were followed).

Benefits:

The idea behind separating the previously mentioned two groups:

General options are options which are auxiliary in nature and do not branch off into multiple separate options, they are less complex
Commands can have optional arguments and their functionality can be extended since their nature is more complex (e.g. starship configure --local where --local is an optional argument)

In terms of functionality, users can:

ping the remote server to see if their configuration is correct and they receive a valid response
configure their API connection by either simply setting their tokens or setting all their configuration details
run their blackbird scripts with the results either appearing on the standard output or can be written to a specified file

Possible Drawbacks: To have General options appear when the help menu is queried, the protected member _optionals of the ArgumentParser class was set. This was the best solution to be found for this task; caution must be taken in case the implementation of the ArgumentParser class changes.

Related GitHub Issues: N/A

XanaduAI / strawberryfields

Refactoring the starship CLI and adding unit tests #312

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