SkyCoord str should not be like repr, but a short nice summary

[mhvk]

This issue is based on PR #3315, which got bogged down because we had not clearly defined what we wanted. From that issue, it seems there is some agreement that Time is a reasonable model, and that:

• __str__ should use to to_string()
• Any latitude needs to always show the sign
• The output needs to contain units
• The output should have the representation that was put in by the user (e.g., Cartesian, not auto-conversion to something else).
• Ideally, the output keeps a format input by the user