Remove UR:file:// and UR:ftp:// from ref search path, plus REF_PATH to EBI

[jkbonfield]

While use of the EBI refget server was originally encouraged by the CRAM inventors, it became a self-imposed DDOS and it is now unreliable due to explicit rate-limiting by the EBI. This removes EBI as a fallback when REF_PATH has not been set.

In doing this we discovered that we could still retrieve references (ironically also from EBI due to the test being a 1000genomes CRAM) via the SQ UR: tag supporting remote URIs. This behaviour is explicitly listed as not being supported in the samtools manpage and we believe it was an accidental ability added when switching from fopen to bgzf_open for reading the UR reference file.

Note this check must be in cram_populate_ref and not load_ref_portion or bgzf_open_ref as the user still has the ability to explicitly request an external reference, eg via "samtools view -T URI".

open_path_mfile() now takes an extra 'int *local' argument which is filled out with non-zero if the file found in REF_PATH is local. Non-local files will be cached to REF_CACHE if set, but it no longer has a default value as we did when ebi refget was the default REF_PATH. This means it should operate much as before, except for the lack of EBI defaults.

[daviesrob]

As this may make failing to get a reference more likely, a useful addition would be to make the

[W::cram_get_ref] Failed to populate reference for id 0

message more useful. It should at least give the name for the offending reference, and maybe also a link to the documentation we plan to add on how to get references.