1.7.21 Added data.table dependency. Much faster, mem-efficient import of usearch and legacy QIIME.

CHANGES IN VERSION 1.7.21
-------------------------

USER-VISIBLE CHANGES

	- Updated `import_qiime` doc to emphasize it is intended for legacy
QIIME files.

	- Much faster and mem-efficient import of legacy QIIME and usearch
files.

	- Uses data.table syntax to better manage import of large files.

	- Entire HMPv35 now imports in about 1 minute, low risk of mem-swap.

	- Added dependency to data.table
1.7.21 Added data.table dependency. Much faster, mem-efficient import of usearch and legacy QIIME.
Merge pull request #304 from joey711/faster-import-uc

1.7.21 Import data.table. Faster, mem-efficient import usearch & legacy QIIME.