Fixed numerous bugs, mainly in CLI operation

I noticed the version you can download in Ubuntu from your cogsci repository is quite old. In the meantime I have made a number of fixes and improvements:

- when writing to an xls(x) file, each cell is given the correct data type, instead of being of type "string" by default
- The CLI part wanted to update the progress bar of the GUI which, naturally, is not available in CLI mode.
- More descriptive error messages are given when incorrect arguments are passed to calls to datamerger from CLI
- Fixes of other minor bugs that caused the output of datamerger not to be what it should.

Could you update the repo version with this one? Merci!