Possibly reduce artifacts size

[biona001]

Knockoffs.jl will soon become a dependency for GhostKnockoffGWAS, which will be compiled into a relocatable app.

Knockoffs's dependency LowRankApprox.jl depends on FFTW.jl which in turn depends on MKL_jll.jl which is a staggering 596.668 MiB. Can we remove this dependency?

[biona001]

We use LowRankApprox.id function which is used in Knockoffs.id_parition_groups to compute group membership. Thus, our option include:

1. Depreciate support for Knockoffs.id_parition_groups (this is reasonable since for most practical purposes we had used Knockoffs.hc_parition_groups
2. Copy relevant code of LowRankApprox.id into the current package (probably not a good idea)

[biona001]

For future reference: from Knockoffs.jl v2.0.0 onwards, we dropped support for id_partition_groups, i.e. option 1 above