itsdb function to check if a table is non-empty

[goodmami]

When reading tables using ItsdbProfile.read_table(), it will raise a KeyError for a table not defined in the relations, raise a ItsdbError if the table is defined, but there is no corresponding file, or return a list of rows for a present, defined file (an empty list for an empty file). Furthermore, because files can be plaintext or compressed, to check if a file is present is a bit clunky:

filepath = os.path.join(p.root, tablename)
if os.path.exists(filepath) or os.path.exists(filepath + '.gz'):
  ...

Similarly, to check if the file is non-empty:

if (os.path.exists(filepath) and os.stat(filepath).st_size > 0) or (os.path.exists(filepath + '.gz') and os.stat(filepath + '.gz') > 0):
  ...

These would be more convenient if the ItsdbProfile class had some table_exists() and table_size() (or table_nonempty() or something) methods. In general, the user should not have to manage the filepath and filepath + '.gz' alternations themselves.