Use web history for large NCBI queries to enable downloading of >300 samples

[zachary-foster]

Currently, the use of rentrez::entrez_search requires retmax = 300 or we get this error:

Error in entrez_check(response) : 
  HTTP failure 414, the request is too large. For large requests, try using web history as described in the rentrez tutorial

Need to either add a way to loop rentrez::entrez_search or use seb history.

Should also add a global parameter to limit the maximum number of genomes/reads to download.

[zachary-foster]

done