Permit detecting an element / attribute was explicitly set (user) or implicitly set (conversion / default value)

[EricCousineau-TRI]

From discussion on 2020-01-23 on Asana.

Notes:
Default values

• Know if a property was actually defined
• libsdformat, in its conversion phase, substitutes in default values
• Odd / non-strict value parsing (e.g. colors, RGBA)
• API: Could add an attribute check, IsExplicitlySet() or UsedDefaultValue() or something, and have overloads for querying that uses optional<> or returns a flag
• May be hard to check if a default value was actually set?
• Could be useful for deprecating attributes that a user set?

\cc @SeanCurtis-TRI

[EricCousineau-TRI]

\cc @marcoag

[azeey]

Waiting for #575 (comment) to be resolved before closing.

[marcoag]

Resolved: #575 (comment). Closing this issue then.