Implement rhpca

[gregorgorjanc]

@sakurita17 is working with @hannesbecher on adding support for genomic data by leveraging the Colleau's indirect method for Hx https://gsejournal.biomedcentral.com/articles/10.1186/s12711-017-0363-9

We will call this rhpca which will stand for randomised hybrid PCA, hyrbrid as in combination of pedigree and genomic data.

Some tasks

• implement rhpca as a new function
• document the function
• unit test the function
• test the functionality with large simulation (from NrfTwin/GenoSim)
• write a vignette with a simple example
• enable work with metafounders (will open a new issue for this as it will need some additional work - see Enable work with metafounders #35)
• find an efficient way with large genotype datasets (this will also be punted to a later stage - see Find an efficient way with large genotype datasets for rhpca #36)

Anything more?

[gregorgorjanc]

CC @sakurita17