added routines output (incomplete) second order elements in BDF

SAME REMARK AS WITH UNV: PLEASE TEST THE NODE ORDERING

added full support for incomplete second order elements (8-node quad,
20-node hexa, 15-node prism and 13-node pyramid):

- second order generation algorithm can now be told to generate either
  complete or incomplete elements

- MSH I/O routines can deal with all elements

- UNV I/O routines now support all the elements in the I-deas spec (I did
  my best to follow the spec for the ordering of the nodes, but I might have
  made some mistakes: PLEASE TEST IF YOU HAVE ACCESS TO A CODE OTHER THAN
  GMSH THAT CAN READ/WRITE UNV FILES)

- Use correct vertex ordering for second order simplices in UNV format
(thanks to Romuald Conty <conty@phimeca.com>)

- MLine2 -> MLine3

- finished Nastran BDF implementation (read+write in all BDF formats:
  free field, small field & long field)
- improved UNV and Medit import
- added detection of bad vertex indices in all import routines
- fixed bug in 2nd order quad constructor

- changed UNV output to match MSH output w.r.t. physical groups
- new UNV file reader

add CBAR for lines in BDF

Added support (both read and write) for the Nastran Bulk Data File
format (BDF).

This might need some adjustments depending on what the actual spec
is: I just reverse engineered some BDF files I found on the web :-)

set the 2 padding bytes in binary STL to 0 (it's just cleaner)

Implemented new binary MSH file format (both read and write).

A 250Mb mesh file (6 million tetrahedra) is now read or written to disk in
less than 10 seconds (on my powerbook's slow hard drive).

added binary stl export