RFE: Add "apparent magnitude passband" as argument to Besancon class

I know I can pass the keyword _iband_ to the class in order to change the passband for the query, for example _iband_=8 means apparent magnitude passband = K (default is filter V, which is _iband_=1), but it would be nice to have passband = V by default so we can change it to K or whatever.

I came across this because I wanted to simulate a color-magnitude diagram K vs (J-K) but I realized that the default passband was V and I had to figure it out what POST keyword and value to use.

Start by reading the Besancon class and tracing how its existing `iband` keyword is passed to the query; the issue does not name a file or test. Clarify whether the requested change is a named passband argument or simply clearer access to the existing keyword, then verify that the default remains V and users can select K or another passband.

Written by the indexing model from the issue text.