Data tab improvements

[jslanini]

1. POR selection-years box should be aligned directly below years check box, rather than with "POR".
2. Should Fill NaN's be default? or do we want users to make a conscious decision to fill missing data.
3. Does update data do anything different than import data? if not, remove.
4. Import dataset-why is import csv/xcel in a different location than import web data? All data importation should be in the same location.