Provide function to extract the value from a dual tensor

[KnutAM]

trafficstars

See the updated docstring for example and motivation, and this example, where the internal _extract_value is used, is a "real-world" use case.

Tensor.extract_value is defined (exclusively for AbstractTensor input) and documented in this PR. For scalars ForwardDiff.value should be used.

Due to the relatively generic name and fairly advanced (and not very frequent) use case, I think it is better to not export it, making it very clear that it is exclusively for tensors.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (258540f) 96.91% compared to head (2309e42) 96.91%.

:exclamation: Your organization needs to install the Codecov GitHub app to enable full functionality.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #208   +/-   ##
=======================================
  Coverage   96.91%   96.91%           
=======================================
  Files          17       17           
  Lines        1490     1492    +2     
=======================================
+ Hits         1444     1446    +2     
  Misses         46       46           

Files	Coverage Δ
src/automatic_differentiation.jl	99.11% <100.00%> (+<0.01%)	:arrow_up:

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.