Spinachboul update function

[Spinachboul]

Changes Made

• Explained Tensor Product Functions for the user's reference.
• Changed the function into a more readable form.
• Attached the relevant plots for different Surrogate Models for visualization.

[codecov[bot]]

Codecov Report

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

Comparison is base (aa744e5) 78.12% compared to head (688fee1) 78.12%. Report is 13 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #457 +/- ## ======================================= Coverage 78.12% 78.12% ======================================= Files 23 23 Lines 3154 3155 +1 ======================================= + Hits 2464 2465 +1 Misses 690 690 ```

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[Spinachboul]

@ChrisRackauckas and @sathvikbhagavan Please have a look at the changes made in the readme file and suggest anything which could be improved!!