Refactoring upon hands on

• All solutions have been reworked so that declarations and definitions are separated.
• Two optional exercises that were problematic have been removed (#125 (closed))
• 3 new exercises have been added (one is the reworking of one deleted in #125 (closed))

@jediaz I am not sure I have fixed all remarks stemming from previous MRs; I'll check that tomorrow.

@vrouvrea I put @jediaz as reviewer but feel free to chime in; in peculiar I would like to make sure you're ok with new exercises (34, 38 and 43).

Closes #124 (closed), #125 (closed)

requested review from @jediaz

assigned to @sgilles

mentioned in merge request !95 (closed)

added 4 commits

• 017ecc3c - #124 (closed) Typo.
• 7ce6021d - #124 (closed) Typo.
• 5b66316e - #124 (closed) English mistake.
• 676f56f1 - #124 (closed) English.

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added 4 commits

• 9d72ad63 - Hands-on solutions: add a comment about noreturn field.
• 9c53705b - #124 (closed) Following Vincent suggestion, use more modern way to silence warnings about unused variables.
• b7a88524 - #124 (closed) Exercice 44: add missing include, use the proper main, and fix newline warning.
• d7b65e86 - #124 (closed) Also replace the static_cast<void> in notebooks.

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added 1 commit

• 92383d49 - #124 (closed) Extra indent.

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added 1 commit

• 4e35d963 - #124 (closed) Use accessor instead of direct data attribute.

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added 3 commits

• b4f877f8 - Fix wrong argument numbering in solution, and add missing quote.
• 3164cdb0 - Add Doxygen comment for a constructor parameter.
• 76c9e93c - Revert commit 4e35d963 that was ill advised:...

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