A rearrangement and renaming of parts of the releases directory. These
changes are to reflect the intention, going forward, to maintain invariant
tagged releases. In future point releases such as 4.0.2 (and later) will
remain invariant once tagged. For each main release (in this case 4.0),
associated point releases (4.0.2, 4.0.3 etc.) will be located in a
subdirectory (i.e. r4.0/). Each main release subdirectory will contain an
evolving bug-fix branch (r4.0-HEAD) on which subsequent bug-fixes may be
committed and from which future point releases can be made.

These restrictions are introduced too late for release-4.0.1 and earlier
releases so these are left at the top level. Thus the arrangement of the
releases directory goes from:

releases --+-- basic_stats.sh

| CMIP5_IPSL
| release-3.4
| release-3.6
| release-4.0
| release-4.0.1
| release-4.0-HEAD
| release-4.0.2

to:

releases --+-- basic_stats.sh

| CMIP5_IPSL
| release-3.4
| release-3.6
| release-4.0
| release-4.0.1
| r4.0 --+

| r4.0-HEAD
| r4.0.2

where the last two directory names have been shortened from release- to r.
Future main releases will involve creating the subdirectory and HEAD branch
with an immediate .0 tag branch.