Fix argument name error for `read_bands` in the CASTEP calculator introduced in !3349

In !3349 (merged) I mistakenly forgot to fix the argument name for read_bands in the CASTEP calculator. As the result in the present master the CASTEP calculator does not run. The present MR fixes this.

(cf. Somehow related to this, for nearly two months the results of the CASTEP calculator cannot be printed in the .xyz file due to the issue I described in #1464 (closed), for which I have proposed a solution in !3308 (closed). I believe this is serious enough and wish to be reviewed as well.)

Checklist

• I am familiar with ASE's contribution guidelines.
• Doc strings in code changed in this MR are up to date.
• Unit tests have been added for new or changed code.
• Issue is resolved via "closes #XXXX" if applicable.