Squeezed F values

[manuGil]

I noticed some undesired behaviour regarding the Sobol sampling. Basically, if we look at the Example 1 in the pull request mentioned above, 'Fs' is sampled with Sobol in 3 dimensions (F11, F12, F22), but in the final doe, these dimensions become squeezed to a single one - F1s, (I believe that happens in 'create_combinations' in doevars.py line 102), and as a result the number of doe points goes from desired 5 to 5 * #dim = 15 in this case, and the overall # of doe is 270, but should be 90. In other words, I think these F11, F12, F22 should appear each in a separate column instead of squeezing them together. Maybe some multi-indexing from pandas could be a solution.