Fix bug in metric tensor elements

[unalmis]

Fix bugs in g^rt_r and g^rz_r and maybe others.
Changes are just copied from github pull request #556

One bug on master is that

DESC/desc/compute/_metric.py

Line 834 in 6b366f5

* dot(data["e^theta"], cross(data["e_zeta"], data["e_zeta_r"]))

should be
$$e_{\theta} \times \partial_{\rho} e_{\zeta}$$
There might be more bugs on master, but I stopped looking after I found the above because they should be corrected by the changes in this PR.

[codecov]

Codecov Report

Merging #628 (ef064ee) into master (6b366f5) will increase coverage by 0.01%.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #628      +/-   ##
==========================================
+ Coverage   94.37%   94.38%   +0.01%     
==========================================
  Files          77       77              
  Lines       17898    17898              
==========================================
+ Hits        16891    16893       +2     
+ Misses       1007     1005       -2     

Files Changed	Coverage Δ
desc/compute/_metric.py	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes