refactor(core): improve grid building routines

[imrn99]

1. remove a useless if / wrong range in the second construction loop
2. fuse and flatten both construction loops, removing redundant computations at the same time
3. replace link usages with set_beta usages
4. set the last assertion to debug only (it can be removed entirely I think).

performance changes for a 512x512 grid (exec time in ms):

Grid type	Original	new fetch_	Optim 1, 2, 3	Optim 4
Square	157.11	89.83	92.98	24.54
Triangle	246.48	120.16	121.79	35.54

[codecov-commenter]

Codecov Report

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

Project coverage is 82.60%. Comparing base (d79671f) to head (fad1ca0).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #209 +/- ## ========================================== + Coverage 82.04% 82.60% +0.56% ========================================== Files 41 41 Lines 5681 5865 +184 ========================================== + Hits 4661 4845 +184 Misses 1020 1020 ```

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[imrn99]

I was just wondering if splitting the loop into (the interior domain and then the boundary (last row & last column)) can not make the code easier to read (and perhaps more efficient without a conditional in the middle).

I managed to reorganize the code by splitting like you suggested. I also extracted some common functions which should help with readability

It does not seem to make much of a difference performance-wise (maybe 5% better? but laptop numbers aren't really reliable)