order_at function imporved. Changes are same as done in merge_cds
order_at improved in rde.py
Improved order_at function in rde.py. Removed cds.py
Edited the comments in the code
Taste cases added for testing order_at for different degrees.
Merge pull request #7237 from anurags92/merge_cds_is_deriv

Improve the efficiency of order_at in rde.py
Add Anurag Sharma to the AUTHORS/aboutus. Welcome to SymPy!
Merge branch 'master' into shift-issue-numbering