ggsashimi can easy read from remote BAMs if allowed to do it

[diekhans]

New feature

Allow ggsashimi to access BAMs remotely via HTTP like many other tools operating on regions of BAMs.

Usage scenario

I want to render images on my laptop so I can look at them immediately, however downloading large BAMs just to render small ranges of the BAMs.
This is inefficient of my time.

Suggest implementation

pysam can read BAMs remotely by HTTP,. however ggsashimi disallows it by checking if there is a file present. Just deleting these two lines and ggsashimi works find on HTTP URLs to indexed BAMs

                if not os.path.isfile(bam):
                        continue

This check is also problematic as it causes ggsashimi to ignore BAMs without telling you which one is bad or why. Just let pysam generate an error.