mv constants from mintpy.objects to mintpy level

[yunjunz]

Description of proposed changes

• move the constants from the objects directory to the root directory, to simplify its usage.

• update all existing usage of mintpy.constants sub-module

Reminders

• Pass Codacy code review (green)
• Pass Pre-commit check (green)
• Pass Circle CI test (green)
• Make sure that your code follows our style. Use the other functions/files as a basis.
• If modifying functionality, describe changes to function behavior and arguments in a comment below the function declaration.