Tutorial: Fixes for Exercise 3

[statkute]

The PR fixes the issues with the exercise 3 from the tutorial:

• There was an inconsistency between the code in the solution vs the exercise, as the exercise used @classmethod and the solution did not. Now that's been made consistent
• The 2nd solution did not lower the memory usage. Not sure why it was added there - the PR removes it
• An edit to the for consistency to utilise the same global variables instead of magic numbers

[codecov-commenter]

Codecov Report

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

Project coverage is 93.02%. Comparing base (41248ed) to head (a72f3aa). Report is 100 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #641 +/- ## ========================================== + Coverage 92.55% 93.02% +0.47% ========================================== Files 91 94 +3 Lines 11304 11415 +111 Branches 1581 2092 +511 ========================================== + Hits 10462 10619 +157 + Misses 837 796 -41 + Partials 5 0 -5 ``` | [Flag](https://app.codecov.io/gh/bloomberg/memray/pull/641/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=bloomberg) | Coverage Δ | | |---|---|---| | [cpp](https://app.codecov.io/gh/bloomberg/memray/pull/641/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=bloomberg) | `93.02% <ø> (+7.08%)` | :arrow_up: | | [python_and_cython](https://app.codecov.io/gh/bloomberg/memray/pull/641/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=bloomberg) | `93.02% <ø> (-2.70%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=bloomberg#carryforward-flags-in-the-pull-request-comment) to find out more.

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