[MRG] Center gradients for mass of emd2 and gw2

[rflamary]

Types of changes

Motivation and context / Related issue

Fixes #364

How has this been tested (if it applies)

PR checklist

• I have read the CONTRIBUTING document.
• The documentation is up-to-date with the changes I made (check build artifacts).
• All tests passed, and additional code has been covered with new tests.
• I have added the PR and Issue fix to the RELEASES.md file.

[codecov]

Codecov Report

Merging #363 (ad30b66) into master (ac4cf44) will increase coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #363      +/-   ##
==========================================
+ Coverage   93.74%   93.78%   +0.03%     
==========================================
  Files          23       23              
  Lines        5902     5902              
==========================================
+ Hits         5533     5535       +2     
+ Misses        369      367       -2