Add function 'create_bgen_files_for_saige'

[richelbilderbeek]

From Marcin Kierczak:

For those of you using SAIGE and PLINK2 to create bgen files. By default PLINK uses 16bit representation for genotypes. SAIGE expects 8bits. To remedy this problem, --export bgen-1.2 bits=8

[richelbilderbeek]

From Hadrien Gourle:

one more warning though, even with bits=8 the bgen from plink are not zlib compressed so it's gonna fail with some programs still (looking at you LDpred2)