update tuple outputs so they will be treated as single-value traits

[HaleySchuhl]

Is your feature request related to a problem? Please describe.
@kmurphy61 requested that we change the data handling of tuple type observations, such as center_of_mass and ellipse_center, so they get output as single-value traits. e.g. ellipse_center_x and ellipse_center_y

Describe the solution you'd like
Update the add_observation step within pcv.analyze.size to save these traits out as single values.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
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