sequtils: robust cnum generation

* Implements more robust cnum generation by not assuming that, if only
  1 cnum is found in the seqSTORE with a given base, it doesn't already
  have version as a suffix.

Signed-off-by: Samuele Kaplun <samuele.kaplun@cern.ch>
1: D100 Docstring missing
4: I102 copyright year is outdated, expected 2014 but got 2012
28:1: E302 expected 2 blank lines, found 1
28: D101 Docstring missing
32: D203 Expected 1 blank line *before* class docstring, found 0
32: D204 Expected 1 blank line *after* class docstring, found 0
32: D400 First line should end with '.', not 'r'
32: D200 One-line docstring should not occupy 3 lines
38: D400 First line should end with '.', not 'm'
50:27: E127 continuation line over-indented for visual indent
52: D401 First line should be imperative: 'Return', not 'Returns'
52: D400 First line should end with '.', not 'd'
97: D401 First line should be imperative: 'Check', not 'Checks'
97: D400 First line should end with '.', not 'e'
97: D200 One-line docstring should not occupy 3 lines
104: D401 First line should be imperative: 'Insert', not 'Inserts'
104: D400 First line should end with '.', not 'E'
104: D200 One-line docstring should not occupy 3 lines
111: D401 First line should be imperative: 'Populate', not 'Populates'
111: D400 First line should end with '.', not 's'
111: D200 One-line docstring should not occupy 3 lines