change new post API to allow multiple meshes in a single view (different
meshes in different steps)

- temp fix I/O to allow skipping $xx fields (this will be common
  with new post-pro)
- prune MElement (move mesh-only stuff in mesh algos)

selective loading of steps)

- notion of partitions and interpolationScheme
- move string utils in separate file

introduce notion of entities in PViewData (in prepa for gmodel stuff)

- clean up GModel API (xxByTag -> getXx, numXx -> getNumXxes, ...)

airplane work: more work to complete remaing features (read, combineSpace,
combineTime) in new post-pro

* re-added all view output routines (POS, STL, MSH, TXT)