update version and changelog

CHANGELOG

+# Version 0.2.1 (May 14, 2024)
+
+* rework export mechanism 
+* change `Operator::computeOutputDims()` with `Operator::forwardDims()`
+* automatic docstring decorators for python
+* add implementation of Operators only performing data/format manipulation
+* add many assertions
+* bind Database and Scaling_Op
+
+# Version 0.2.0 (April 11, 2024)
+
 # Version 0.1.1 (January 29, 2024)
 
 [Add] Support of a negative value for Reshape Operator shape attribute.

version.txt

-0.2.0
+0.2.1