refactor common code that teases numbers from strings in table or CSV input

[DrPaulBrewer]

These are mostly notes I am making to myself, but if anyone wants to comment, go ahead.

affected functions:

• shared.parseCSV

• `shared.fromTable`
  

• editor()'s private func onCellChange()
• If meta data were created about header rows on CSV object creation (at begin or after reading a line in fetch) then the code would have better direction of when and in what columns to tease numbers from strings. However, not all data has a header row. Some data has rows whose column types, number of columns, and meanings vary from one row to the next and I think it is desirable to support arbitrary comma separated data, including non-square data.
• There is duplication of need to tease numbers, or null/undefined in input from tables and input CSV files and similar code in shared.parseCSV and shared.fromTable. code in onCellChange() is a little simpler.
• In the shared routines cells are set to null or undefined when encountering a string "null" or "undefined".
• In light of how JSON-serialization works, setting anything to undefined is probably a bad idea. undefined does not JSON-serialize across bare values, objects, and arrays (at least on chrome, the results are undefined, an omitted key:value pair, and a null element and the last two are obviously not invertible). JSON is used in the interactions with Storage and future ajax.
• The goals should be: tease numbers from strings consistently in user, table, or file input; handle empty or null elements in a way that makes sense; avoid use of undefined as it tends to create inconsistencies

Question whose answers should be determined before refactoring:

• When, if ever, should null be converted consistently to a blank string or a blank string to null or anything else to/from null when reading or writing data? Should null be avoided completely and all CSVs be in terms of string/number?
• If something is undefined or the string "undefined", what should happen? Perhaps string "undefined" or even string "null" is valid data in some use cases.