dataMgr:importer/whImporter: reorganize the code structure

1) Remove 'dataFile' and 'dataTable' arguments from __init__(s) and
   change to specify them in class methods instead to extend reusibility.
2) Remove useless setAttrs() because of 1).
3) Change 'partReimportTo()' to 'partReimport()'.
4) Add getRecordField() to return a specified field from a record.
5) Add importFromDir() method to import data records from a directory.

Signed-off-by: Zhengwang Ruan <ruan.zhengwang@gmail.com>