be tolerant with segnum language

[Glottotopia]

In line 420 of flex2csv.py you have

...
   (f"segnum_{conf['gloss_lg']}_phrase", "Sentence_Number"),
...

I do not know how often there is a different "segnum" per language in flex files. In my case, the segnum language was set to French, and the value was "3.1", so not particularly localized. The program failed hard and produced no output.

I can of course prepare some yaml file to take care of this, but maybe one could in the case of only one segnum being present for a given sentence simply default to that segnum instead of requiring the language to be specified.

[fmatter]

I've run across this issue, too. I'll take a look at it if I find the time.