improve installation of quast, spades

[agarwal]

There are a few ways the quast package could be improved:

• Separate build versus install. Opam has for some time supported a separate install phase, and install steps are recommended to go there. Right now opam-build.sh does both, so this file could be split into two. But also, next points suggest avoiding the shell script altogether.
• Can we assume users of packages installed with opam will have opam installed? Seems like a reasonable assumption unless you're planning to move a generated .opam directory to a different computer. If so, the script quast that is being generated could be a static file; replace $PREFIX with opam config var prefix.
• Can the .install file be generated in advance? The new biopam-dev tool may help. I'm moving towards statically generating .install files as much as possible. It makes it more transparent what will be done. I'm even providing different install files for different OS's, e.g. see how the updated ncbi-vdb package does it.

[agarwal]

The comments about quast also apply to spades. We should settle on some packaging best practices and document them.