Reduce functions in namespace

[bschilder]

A lot of functions that most people wouldn't use in practice are exported. This crowds the namespace and makes it harder to find key functions.

Making functions that only ever get called internally as internal.

[bschilder]

The following are now internal (not exported to NAMESPACE). I made sure none of these were called in the vignettes, README, or @examples.

• [x] map_specificity_to_entrez
• [x] create_top10percent_genesets_file
• [x] standardise.sumstats.column.headers.crossplatform
• [x] standardise.sumstats.column.headers
• [x] format.sumstats.for.magma.macOnly
• [x] load_snp_loc_data
• [x] build_snp_location_tables

[bschilder]

Avoiding exporting of all ggplot2 functions also drastically reduces the namespace. Requires prefixing all functions with ggplot2:: but worth it.