Support AutoSQL

[superbobry]

BigBed files can store extra fields in addition to the predefined BED fields. If you add extra fields to your bigBed file, you must include a .as (AutoSQL) format file describing the fields.

Motivation and details on AutoSQL are described here. At the moment we've decided to restrict big to predefined BED fields, but we'll have to deal with AutoSQL at some point.

[iromeo]

2017-11-09 17:35:29] BigFile [WARN] AutoSQL queries are unsupported
Logging is too aggressive here, e.g. it spoils epigenome project log file because epigenome browser opens big file on each rendering event.

I think it is better to log it as "trace" info or even switch warning off.

[superbobry]

Yep, just switch it off. This AutoSQL thing is a mess, I doubt we would ever want to implement it.