BGLR support for sparse design-matrices

BGLR is a great package with rich features. I only missed one: support for sparse design matrices.
Therefore I added a C-function that samples the regression coefficients just like "sample_betas" but
for compressed column-major sparse matrices (R-class: "dgCMatrix").
I didnt change any behaviour, algorithm or statistics - it is just about the computational part.
I only added minor changes to the R-function 'BGLR' - it was just necessary to make every step aware of
possible sparse design matrices.

For me this addition makes BGLR really complete. It allows us to also solve sparse problems, like
pedigree-based analysis or the inclusion of very sparse grouping-factors.

Thank you for this excellent package and I hope you consider the addition!

Best regards,

Claas Heuer