Option to avoid saving irreps.dat

[gersonjferreira]

The function write_characters(...) at kpoint.py writes the irreps identification to this file by default and there's no option to change its name or opt to not create this file.

I'm running your code directly in python as a library, without using the CLI interface. I do this because I want to use the output of your code to do other calculations. So, I would prefer no to write the irreps.dat file to disc.

[stepan-tsirkin]

@gersonjferreira , you are welcome to introduce a tag to chose the name of the file or disabling the writing of the file, and make a PR.

[gersonjferreira]

Thanks. I'll do it, and I'll try to meet your coding style as much as possible.