Added the first proof of concept for the virtual table (vtable)
Added full support for slices in vtables
The metadata info for columns drops the paths.

Having the complete paths in the dirs metadata prevented a safe
operation when moving the tables to different directories,
or just when loading tables from different places that they where
created.

One issue that appeared while doing the fix is that, when a new
column in-memory is inserted, it is not clear which should be the
default action (copy on-disk or just use the reference?).  Probably
we would need to setup a configuration file for specifying default
actions.  For the time being, the reference will be used.
Merge branch 'master' into vtable
Syncing vtable branch with master
Added support for reading all BLZ files in a single directory
Added support for querying vtables
numexpr_here is not longer outside the module
Getting the numexpr functions in scope
Fixed a case when arrays where zero-length
First attempt to reduce the amount of memory used in barray caches
Better check for directories
Merge branch 'vtable'