Introduce a default for embed/embed! and retraction by projection

[kellertuer]

update the default for vector transport by projection, which now can also include a embed (since we have a default).

[codecov[bot]]

Codecov Report

Merging #99 (e6474c1) into master (4341444) will decrease coverage by 0.04%. The diff coverage is 100.00%.

:exclamation: Current head e6474c1 differs from pull request most recent head f59eb54. Consider uploading reports for the commit f59eb54 to get more accurate results

@@            Coverage Diff             @@
##           master      #99      +/-   ##
==========================================
- Coverage   99.80%   99.75%   -0.05%     
==========================================
  Files          17       17              
  Lines        2049     2046       -3     
==========================================
- Hits         2045     2041       -4     
- Misses          4        5       +1     

Impacted Files	Coverage Δ
src/ManifoldsBase.jl	97.67% <100.00%> (-1.54%)	:arrow_down:
src/vector_transport.jl	100.00% <100.00%> (ø)
src/ValidationManifold.jl	99.48% <0.00%> (-0.52%)	:arrow_down:
src/numbers.jl	100.00% <0.00%> (ø)
src/PowerManifold.jl	99.77% <0.00%> (+0.44%)	:arrow_up:

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[kellertuer]

One could of course discuss whether just projection would be a good retraction, but by default we do not know much more about the embedding we project from (a lot of them have a plus but maybe not all).