Make one single library

[lilastic]

Currently, one has to import header file 'xlsxio_write.h' and link with shared library 'libwrite*' to write XLSX files and import header 'xlsxio_read.h' and link with shared library 'libread* to read XLSX files.

While this segmentation has its merit (logic-wise), in practice, it's better to have everything under just one header file and one shared library. This alleviates the developer and user of the xlsxio library.