several fixes and renames

- 'axes_mapping' -> 'axes_map'
- fix wrong init of _axes_map
- some changes to coord_sys behavior
- fix wrong init of _coord_sys
- add copy() method
- fix curve() function
remove axes_map argument in __init__; add pixel_size() and pixel_area() methods
remove axes_map argument in __init__
remove axes_map argument in __init__; make 'lab' default system
fix no-return value issue; rename 'position'->'location'
WIP