Delay warning of missing requirements in io.cif as a late as possible

[sphuber]

Is your feature request related to a problem? Please describe.
Delay the printing of warnings as late as possible in io.cif.
Currently, just importing the module io.cif will print the warning if pybtex is not installed:

try:
    from pybtex.database import BibliographyData, Entry
except ImportError:
    warnings.warn("Please install optional dependency pybtex if you"
                  "want to extract references from CIF files.")
    BibliographyData, Entry = None, None

However, the only function requiring it is get_bibtex_string.

Describe the solution you'd like
Since these warnings are not easy to suppress and show up even when not applicable it would be better to just move this warning to within the get_bibtex_string itself. One can simply extend the message of the requires decorator

[shyuep]

Thanks. I fixed this. But instead of a warning, I simply make it throw an exception. The idea is that if someone is calling get_bibtex_string, they must actually be interested in a result and a runtime error should ensue if pybtex is not installed.

[sphuber]

Thanks for the very quick response and action @shyuep 👍