add group ID and strand columns to expression.bed.gz and splicing.bed.gz

[jackhump]

GTEX pipeline produces BED files formatted for FASTQTL, not QTLtools. The only difference is that FASTQTL BED files have 6 columns before the per-sample columns:

Chromosome ID [string] Start genomic position of the phenotype (here the TSS of gene1) [integer, 0-based] End genomic position of the phenotype (here the TSS of gene1) [integer, 1-based] Phenotype ID (here the exon IDs) [string]. Phenotype group ID (here the gene IDs, multiple exons belong to the same gene) [string] Strand orientation [+/-]

These final two columns must be added in the eqtl_prepare_expression.py and the sqtl_prepare_expression.py

[jackhump]

for both sQTLs and eQTLs, group ID must be gene.