Dataset challenges

[alastair]

Dataset challenges, based on work reported at http://mtg.upf.edu/node/3638

[pep8speaks]

Hello @alastair! Thanks for opening this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file challenge_eval/evaluate_models.py:

Line 9:1: E402 module level import not at top of file
Line 11:1: E402 module level import not at top of file
Line 12:1: E402 module level import not at top of file
Line 13:1: E402 module level import not at top of file
Line 14:1: E402 module level import not at top of file
Line 15:1: E402 module level import not at top of file
Line 16:1: E402 module level import not at top of file
Line 17:1: E402 module level import not at top of file
Line 18:1: E402 module level import not at top of file
Line 19:1: E402 module level import not at top of file
Line 20:1: E402 module level import not at top of file
Line 21:1: E402 module level import not at top of file
Line 22:1: E402 module level import not at top of file
Line 23:1: E402 module level import not at top of file
Line 24:1: E402 module level import not at top of file
Line 25:1: E402 module level import not at top of file
Line 125:13: E265 block comment should start with '# '
Line 126:13: E265 block comment should start with '# '
Line 135:1: E305 expected 2 blank lines after class or function definition, found 1

• In the file db/dataset.py:

Line 307:10: E111 indentation is not a multiple of four
Line 307:10: E117 over-indented
Line 314:9: E122 continuation line missing indentation or outdented

• In the file utils/models.py:

Line 7:1: E402 module level import not at top of file

• In the file webserver/admin/views/challenges.py:

Line 88:1: W391 blank line at end of file

• In the file webserver/views/data.py:

Line 133:1: E303 too many blank lines (3)

• In the file webserver/views/datasets.py:

Line 261:21: E127 continuation line over-indented for visual indent