Usability improvements

Several usability improvements have been added:
   - Now there are two sample datasets to download: oil and survey.
   - Error control has been improved in characterization and tests function.
   - Code cosmetic changes.
   - License header added to tests.R file.
   - Shapiro-Wilk test threshold changed for normality tests tab.
   - Batch tests results now are showed as a table.
   - Sample size threshold added to batch tests.