Add `is_flat` function

[mateuszbaran]

This is needed to correctly implement injectivity radius for tangent bundle, which is then needed to fix Manopt line search on tangent bundle.

[mateuszbaran]

Now I just need to figure out for every manifold we have in Manifolds.jl if it's flat.

[codecov[bot]]

Codecov Report

Merging #143 (b724f54) into master (c7e2a17) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #143   +/-   ##
=======================================
  Coverage   99.79%   99.79%           
=======================================
  Files          19       19           
  Lines        2452     2454    +2     
=======================================
+ Hits         2447     2449    +2     
  Misses          5        5           

Impacted Files	Coverage Δ
src/ManifoldsBase.jl	99.55% <ø> (ø)
src/decorator_trait.jl	100.00% <ø> (ø)
src/DefaultManifold.jl	100.00% <100.00%> (ø)
src/PowerManifold.jl	99.62% <100.00%> (+<0.01%)	:arrow_up:

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[mateuszbaran]

It's already used and defined in https://github.com/JuliaManifolds/Manifolds.jl/pull/567 . Is there something unclear?

[mateuszbaran]

I just have to go once again over manifolds there because I didn't yet check if some special cases of parametrized manifolds reduce to something flat.

[kellertuer]

Ah, then maybe I was expecting more. But we can surely do that later when we have the need for principal curvatures (then this function would check if all of them are zero)

[mateuszbaran]

Maybe there will be something more in the future, who knows.