Add casting from owner constructor to basic_observer_ptr

[cschreib]

Fixes #12. This also fixes documentation errors for the basic_observer_ptr constructor.

[codecov-commenter]

Codecov Report

Merging #13 (3f64587) into main (08f687f) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main       #13   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            1         1           
  Lines          295       299    +4     
=========================================
+ Hits           295       299    +4     

Impacted Files	Coverage Δ
include/oup/observable_unique_ptr.hpp	100.00% <100.00%> (ø)

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