Support for const objects -> Code optimization

[AngryMaciek]

Is your feature request related to a problem? Please describe. How should I implement the methods to work on both const and non-const objects?

Describe the solution you'd like Avoid code duplication!

Additional context

• https://www.learncpp.com/cpp-tutorial/810-const-class-objects-and-member-functions/
• https://stackoverflow.com/questions/35798955/when-there-is-no-non-const-operator-overload-will-non-const-object-use-const-op

[AngryMaciek]

The use of const keyword is a little messy throughout the library --- needs cleaning.

[AngryMaciek]

Also here: which class members should return by value and which by reference?

[AngryMaciek]

I askked for clarification @ SO: https://stackoverflow.com/questions/64741595/use-of-const-keyword-and-by-reference-return-in-a-c-class

[AngryMaciek]

TODO:

• [x] static
• [x] const
• [x] code duplication
• [x] pointers vs. [ ]
• [x] values/references
• [x] add additional tests on const objects.

Compare to: https://www.boost.org/doc/libs/1_34_1/doc/html/boost_math/octonions.html

[AngryMaciek]

finished with 7d30765