Update PDL $VERSION to decimal strings

Per discussion on pdl-porters list, we're moving to standardize
version numbers to the single decimal point string representation
as discussed here:

http://www.dagolden.com/index.php/369/version-numbers-should-be-boring/
Update $VERSION to 2.004011_011 for CPAN developers release

First attempt with new version string standard
Fix instructions for intall with new VERSION numbers

The filename of development releases don't have the
underscore in them.
More VERSION related fixes.

Make $PDL::VERSION in Basic/PDL.pm be the new, preferred
string format.  Put the '$VERSION = eval $VARSION' in
top level Makefile.PL when it generates Version.pm.
Forgot our in Version.pm generated file
Update VERSION to 2.004_012 for more development

Note the new standard version scheme.