version 2.1.0

See merge request csst_sim/csst-simulation!16

1. Update the field distortion models for FGS detectors (with new wcs definition). 2. Update README.md

starting point of the new version

See merge request csst_sim/csst-simulation!15

rotate e1, e2.
remove the rotation parameter in Galaxy class (not used before).
modeling of ellpiticity rotation should be implemented in Catalog.py

Straylight model

See merge request csst_sim/csst-simulation!14