Add dbsnp tarball support

[sufyansuleman]

load_snp_loc_data.R

Added support for a dbSNP_tarball argument: if a path is provided, we unpack the package bundle into a temp directory and load the OnDiskLongTable directly from its inst/extdata folder.

Otherwise fall back to loading the installed SNPlocs.Hsapiens.dbSNP.GRCh<37|38> Bioconductor package.

validate_parameters.R

Introduced skip_pkg_checks <- !is.null(dbSNP_tarball) so that when a tarball is supplied we skip the SNPlocs/BSgenome package–installation checks.

Added early validation for a missing or non-existent tarball path.

Moved the numeric‐only dbSNP check outside of the “skip” block, so invalid values always error out immediately.

format_sumstats.R

Passed dbSNP_tarball through into load_snp_loc_data() and all downstream SNPlocs‐dependent checks.

Ensured that when a tarball is provided, neither validate_parameters() nor the loader will try to require an installed SNPlocs.Hsapiens.* package.

[Al-Murphy]

This only covers some of the necessary changes, I'll incorporate the update to load_snp_loc_data() and refactor the rest myself.

Cheers, Alan.