Encode chromosome; avoid RSQLite.

- That is, I changed the encoding of the chromosome to be consistent in
  the expr and geno tables.

- I also changed the database creation tool to just output a text file
  of SQL commands, which are then loaded using the sqlite3 program. This
  seems to be significantly faster than using RSQLite (perhaps it isn't
  possible to turn off autocommit in RSQLite?).