TableHandler improvements

[bangerth]

https://github.com/geodynamics/aspect/pull/1429 shows a number of oddities in the TableHandler class that we've known of for a while already but that are still worth addressing:

• Provide a "canonical" way to start a new row in the table, rather than have some magic in TableHandler::add_value that does this.
• Provide a way to "declare" a column, rather than creating it the first time it is used. This would help with being able to describe a particular order for columns we may want to have when creating output, and keeping this order even if the code that fills the table moves around within a piece of software.

[drwells]

I wrote some (as of yet) unmerged changes in ConvergenceTable (which inherits from this class) so I am also interested in making improvements here. I don't have anything concrete to offer right now but I would be interested in helping overhaul these two classes :)