Add patchversion scripting command

[giacomofiorin]

A NAMD patch release is upcoming, which may also contain bugfixes for Colvars. However, the Colvars version is not being updated in it, which makes it difficult to detect in script if a given bug has been fixed or not.

The new command cv patchversion allows to retrieve the patch version in scripts.

Example use in a NAMD script:

set colvars_version ""
set colvars_patch_version 0
catch { set colvars_version [cv version] }
if { ${colvars_version} != "" } {
    catch { set colvars_patch_version [cv patchversion] }
} else {
    puts stderr "Error: cannot get Colvars version"
}

if { ${colvars_patch_version} > 0 } {
    puts "Colvars version: ${colvars_version} (patch ${colvars_patch_version}): some nasty bugs have been fixed"
} else {
    puts "Colvars version: ${colvars_version}: some nasty bugs are still in there!"
}