Simplify webpage_level12_jpgs and webpage_cols4table

[bhilbert4]

webpage_level12_jpgs lists the suffixes of the jpg file types for which jpg thumbnails are created.
webpage_cols4table lists the columns that will be written to the index.html file.

At the moment, there is very little checking that these two lists are consistent. #41 adds a small check such that any suffixes in webpage_level12_jpgs but not in webpage_cols4table will not crash the code. But checking for the opposite situation, where the user requests a column in the html file for which they did not ask for jpgs, is much harder to check for. And if there is a jpg suffix in webpage_cols4table that is not in webpage_level12_jpgs, the code crashes.

Easier might be to redefine webpage_cols4table to initially not include any jpg suffixes, and within the code, the entire list of webpage_level12_jpgs will be added to the webpage_cols4table list before creating the html file. In that way, a user would be locked in to seeing all of the jpgs they requested, which doesn't seem like a bad thing. But this way it would be guaranteed that webpage_cols4table would not contain jpg suffixes for which jpgs were not created (unless the user places them in webpage_cols4table to begin with...)