Implement riemann_tensor on power manifold

[mateuszbaran]

@kellertuer I think it will be useful for both power and product manifolds :slightly_smiling_face: .

[kellertuer]

Should we maybe add a documentation to riemannian_tensor for AbstractPowerManifold?

[mateuszbaran]

Sure, I will add some docs.

[codecov[bot]]

Codecov Report

Merging #128 (0125133) into master (9d6ca13) will increase coverage by 0.04%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #128      +/-   ##
==========================================
+ Coverage   99.81%   99.86%   +0.04%     
==========================================
  Files          18       18              
  Lines        2188     2199      +11     
==========================================
+ Hits         2184     2196      +12     
+ Misses          4        3       -1     

Impacted Files	Coverage Δ
src/PowerManifold.jl	99.59% <100.00%> (+0.21%)	:arrow_up:

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