Fixes #751 and #882

[lujop]

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Description

Column type detection uses the same logic as before but all column cell types are used instead of only the first one.
If all are the same type it's used, and if not string is used to be able to accommodate all values.

This fixes #751 and with the combination of 74a54aa fixes also #882

Testing

Existing tests were extended to cover new behavior and use cases from #751 and #882

[lujop]

The CI fails seems to be temporary ones due to network problems.
Can someone relaunch the suite?

[lwhite1]

i don't know anything about the Excel reader code, but why not reuse the value-based approach used for text files?

[lujop]

i don't know anything about the Excel reader code, but why not reuse the value-based approach used for text files?
I think that's not a good idea and that current implementation is better.
Excel contrary to other readers provides a type for each cell and current implementation uses it to determine column type.

IMO using the provided Excel type is the best option as is the best intent from the application and the user to define what type a cell is. That takes into account locale options of the user, formatting options, ...
And trying to obviate that info I think that will be confusing and very error-prone for the user.

There are also implementation concerns, in Excel, or at least in POI when you have a cell of type Number you can't get the cell as a String, your best intention is to try to format the number as a string using the format options that the user used
That is what it tries the appendValue function in line 338

tablesaw/excel/src/main/java/tech/tablesaw/io/xlsx/XlsxReader.java

Line 324 in 9a7e18f

private Column<?> appendValue(Column<?> column, Cell cell) {

But it will require a refactor and I think that won't be robust and that not in all cases we will get the same string value that the user sees.

[lwhite1]

i will bow to your expertise. I will merge after the conflicts are resolved.