Excel styles should default to using number format from display.precision option

[jnothman]

Excel output should be as close to the DataFrame html display as possible. Excel controls the display of each cell with a number format style attribute. Pandas controls it with the display.precision option. Excel export should adhere to the pandas option.

[rmhowe425]

@mroeschke I think this PR satisfies the acceptance criteria for this issue.

If we all agree then we can probably close this issue.

[mroeschke]

Thanks. Yes I think this can be closed