Implement helper methods to reduce looping and struct accessor unwieldiness

[anweiss]

When creating OSCAL structs from various data sources, there can often be a number of unwieldy looping and struct accessor lines. Even creating a simple control with a single string narrative requires an unnecessary number of lines of code. Instead, we should provide helper methods that make it easy to go from simple strings to various OSCAL components (e.g. Parts, Prose, etc). So for example:

func NewPart(narrative string) catalog.Part {
	return catalog.Part{
		Prose: &catalog.Prose{
			P: []catalog.P{
				{Raw: narrative},
			},
		},
	}
}

Helpers should be created for reading nested information as well so as to avoid unwieldy iteration constructs.

[asadullah-yousuf-10p]

@anweiss we are mostly using these structs in code generators for now. I was looking for some use-cases where you feel these helper methods are required. This will help us create effective helper methods that will actually be used.

[anweiss]

@asadullah-yousuf-10p the use cases I have in mind are geared towards developers that are working directly with the OSCAL structs but that don't have an already formatted OSCAL artifact that they are parsing. So for instance, if I'm building a tool to convert a generic document into OSCAL format, I'd like to be able to use the OSCAL structs provided by oscalkit. And in that case, helper methods such as the one above would be advantageous.

[minhaj10p]

@anweiss
Let me know your thoughts on this approach #82