implementation of the gips mult

[PrzeChoj]

TODO:

• initial implementation of gips mult
• documentation
• delta is also an parameter that can vary
• implementation of plot.gips()
• gips() cannot receive number_of_observations as a list.
• A little improvement to the documentation, e.g. in gips() the S parameter can be a list. Mention the was_mean_esimated has to be the same for all samples.
• a better validation for compare_posteriories_of_perms() when the perm1, perm2 are not gips.
• Validation of number_of_observations -- >= 1, or >= 2.
• Example of multi-sample in gips vignette

[PrzeChoj]

Let's wait for the gipsDA package to implement and use this branch befor merging to main. Maybe we will find some bugs.

[PrzeChoj]

Authors of gipsDA package assured me this is ok