Description of proposed changes

• asc_desc2horz_vert.py:

  • use a window-by-window strategy to speedup the pixel-wised inversion
  • add get_design_matrix4east_north_up() for future 3D decomposition
  • bug fix for --oo option during file writing

• gps.GPS.read_displacement(): add self.date_list member variable, for easy use

• stack.timeseries: remove get_design_matrix4time_func, as it's not used anywhere in mintpy anymore, to avoid circular imports

Reminders

• Pass Codacy code review (green)
• Pass Circle CI test (green)
• If adding new functionality, add a detailed description to the documentation and/or an example.