Idea for ska version

[taldcroft]

Following discussion in #19, one possibility is to have each meta package provide a version file. Then the ska_version script would effectively paste all those together, giving a possibly long but unique version description of the environment. E.g.

$SKA/versions/
    ska3-flight
    ska3-core
    ska3-pinned
    ska3-perl

Then ska_version might give something like:

flight-2018.09.22:core-2018.08.01:pinned-2018.08.01:perl-2018.08.01

If you decide to uninstall ska3-flight then it looks like core-2018.08.01:pinned-2018.08.01:perl-2018.08.01.

[jeanconn]

ska_version could also be a script that queries the versions of those packages from conda, but version files are probably simpler.

[taldcroft]

Per discussion, implement a ska_version script that gets package names and versions from conda list (JSON file) and looks for any package name starting with ska3-. So this requires the convention of having versioned metapackaged named thusly (fine).

Output looks like below (alphabetized):

core-2018.09.22:flight-2018.08.01:pinned-2018.08.01:perl-2018.08.01

[jeanconn]

Closed by #37