ENH: improvements for nastran surface writer

- select default nastran PLOAD2 or PLOAD4 based on field type.
  Default to PLOAD2 for scalar types and PLOAD4 for vectors etc.

- relocate nastran SHELL/MAT cards.
  Previously wrote at the end of the file, now emit when writing the
  geometry itself. This improves modularity (of code and files)

- initial support for common geometry file for nastran