add a warning message for match cohorts argument to warn about extra columns

[daniellenewby]

if a user add demographics to a cohort then matches the demographics are not carried through to the matched cohort and this can cause problems later on in the analysis as this information is missing.

For example:

1. create a cohort
2. add demographics
3. do matching
4. do characteristics

In the workflow above the matched cohort will have strata information on sex and age etc as step 3 was done after step 2. Needs a warning to warn user that demographics are not carried through for the matched cohort.

Workaround:

1. create a cohort
2. add demographics
3. do matching
4. add demographics again
5. do characteristics

This creates a warning about overwriting columns for the original columns and the sampled columns however if i do this:

1. create a cohort
2. do matching
3. add demographics
4. add some filtering and update the attrition
5. do characteristics

The above is annoying i would get attrition for my original and the matched and sampled ones which i do not want therefore the middle workflow is better.