Refactor/use classes for blocks

[GFabien]

Use S3 classes to handle the different types of blocks. We refer to primal blocks for blocks having more rows than columns and dual blocks in the opposite case. So far, the following types of blocks are handled:

• primal blocks with tau = sparsity = 1 (block),
• dual blocks with tau = sparsity = 1 (dual_block),
• primal blocks with tau != 1 (primal_regularized_block),
• dual blocks with tau != 1 (dual_regularized_block),
• blocks with sparsity != 1 (sparse_block). These different classes of blocks allow the implementations of RGCCA and SGCCA. The former rgccad and sgcca functions have been merged into the rgcca_outer_loop function, and the former rgccak and sgccak functions have been merged into the rgcca_inner_loop function. This last function creates the block objects with their corresponding classes and calls the associated methods to initialize, update, and post-process the blocks.

In addition, it is now possible to specify a custom connection matrix when there is a response block.

[codecov-commenter]

Codecov Report

Attention: 39 lines in your changes are missing coverage. Please review.

Comparison is base (17347b7) 94.39% compared to head (55c51f1) 95.02%.

:exclamation: Current head 55c51f1 differs from pull request most recent head b0cb631. Consider uploading reports for the commit b0cb631 to get more accurate results

Files	Patch %	Lines
R/rgcca_inner_loop.R	72.54%	14 Missing :warning:
R/rgcca_outer_loop.R	81.57%	14 Missing :warning:
R/block_postprocess.R	47.05%	9 Missing :warning:
R/block_init.R	85.71%	2 Missing :warning:

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #77 +/- ## ========================================== + Coverage 94.39% 95.02% +0.63% ========================================== Files 83 80 -3 Lines 3320 3138 -182 ========================================== - Hits 3134 2982 -152 + Misses 186 156 -30 ```

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