Bug fixes for enable_observer_from_this

[cschreib]

Small changes and bug fixes:

• Fixed observable_unique_ptr::reset(T*) incorrectly marked noexcept.
• Fixed enable_observable_from_this not supporting multiple inheritance, or storage in base-class owner pointer.
• Decreased memory usage of enable_observable_from_this.
• Added explicit checks against T[] since arrays are not supported.
• Fixed documentation errors

[codecov-commenter]

Codecov Report

Merging #4 (726c5a7) into main (b697b94) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main        #4   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            1         1           
  Lines          275       285   +10     
=========================================
+ Hits           275       285   +10     

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

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