provide col to df funcions

[JanMarvin]

This pull request provides functions to import <col .../> nodes to a data frame and a function to create a <col .../> node from a data frame. Both functions might be used with loadWorkbook and saveWorkbook. They provide what is required to easily modify <col .../>.

Contrary to rows, columns do not have to be present for each column of a data frame. There can be a single column handling multiple columns.

[JanMarvin]

> cols_df <- openxlsx2:::col_to_df(read_xml(wb$worksheets[[1]]$cols_attr))
> openxlsx2:::df_to_col(cols_df)
> openxlsx2:::empty_cols_attr()
[1] "<col collapsed=\"false\" customWidth=\"true\" hidden=\"false\" max=\"1\" min=\"1\" outlineLevel=\"0\" style=\"0\" width=\"3.45\"/>" 
[...]
> cols_df
  bestFit collapsed customWidth hidden max min outlineLevel phonetic style width
0             false        true  false   1   1            0              0  3.45
1             false        true  false   2   2            0              0 11.33
2             false        true  false   3   3            0              0  7.65
3             false        true  false   4   4            0              0 10.14
4             false        true  false   5   5            0              0  7.65
5             false        true  false   6   6            0              0  9.55
6             false        true   true   7   7            0              0  9.15
7             false        true  false   8   8            0              0  7.65
8             false        true  false   9   9            0              0  9.55