Handle missing values in hierarchy columns

[caitlinch]

If there are missing values in the Hierarchy columns, raise an error/warning message. This issue is likely to arise when data isn't tidy i.e., when notes are placed below data in columns.

Either:

• Raise error saying "Fix this - you have missing values"
  • Error should be above "Estimate prevalence" button. Grey out the "Estimate prevalence" button.
• Raise warning saying "There were missing values in columns A B C, dropped X rows with missing values"
  • Warning should come up before "Estimate prevalence" is pressed, in the side panel

Should follow on from PoolTestR Issue #21