Be more professional in API naming.

[rainwoodman]

Shall we do the following renaming in the API:

BigFile -> Group
BigBlock -> Column
BigData -> DataSet

[rainwoodman]

Python API is updated.

The C-API will probably stay the way it is, because it is not worth it to add a lot of new code to provide the backward compatibility.

Unless we do this in two phases -- first add an-alias to bigfile/legacy that has the old code, then do the breakage and add the new code; and when downstream breaks, they can use the relatively stable legacy API till they decide to migrate with 'search/replace'.