InBamFile::open should throw [not assert] when input is a wrong format

[Peter]

I tried to open a InBamFile? which was not a bam triggering this assertion

../yat-0.10.1/yat/omic/BamFile.cc:78: void theplu::yat::omic::InBamFile::open(const std::string&): Assertion `header_.header_' failed.

I think we should either remove that assertion or replace it with an exception. Compare with std::ifstream::open(1)

[Peter]

I'll go with exception.

[Peter]

(In [3079]) throw if creating bam_header fails, e.g. if file is not a bam. fixes #767