Handling of Plots worksheet when there may be images

Prior to beginning processing, we have limited E3__Plots to six variables because users have the option of providing plot maps that may cause additional phantom variables to be created. The code used was:
E3__Plots <- E3__Plots[ , c(1:6)]
But since we allow flexibility in adding identifiers, we can have more than 6 valid columns.
So we need to somehow test for the valid number of variables in row 4, while ignoring extraneous text.

Locate the processing step that currently limits E3__Plots to columns 1:6, then inspect row 4 to determine how valid variables and extraneous text are represented. Done means E3__Plots preserves all valid identifier columns while ignoring phantom variables and unrelated text.

Written by the indexing model from the issue text.