Resolve "Build info string with .join()"
Closes #920 (closed)
Several files in GPAW build info_strings iteratively, which is suboptimal for the method in which strings are stored. Therefore, these strings were appended to an info_string_list (isl) and joined together using ''.join(isl). Additionally, f-strings were used in place of older '%' based string formatting.
I also corrected some spelling mistakes