Improve coverage and documentation

[GFabien]

This PR aims to

• Improve test coverage;
• Remove rd files for most internal functions, and add documentation to undocumented functions;
• Simplify and homogenize printing functions. Add snapshot tests to print outputs;
• Remove the use of sapply;
• Correct Average Variance Explained (AVE) when components are correlated;
• Refactor prediction model checking and call caret::modelGetInfo in rgcca_cv to avoid multiple redundant calls;
• Fix minor bugs: use of superblock in bootstrap for SGCCA and setting of SPCA.