ENH: make Nastran PLOAD2,PLOAD4 field mappings optional

- PLOAD2 is a reasonable default and PLOAD4 output
  (currently misused for vectors/tensors) is the exception.

- simplify the specification by using optional PLOAD4, PLOAD2 wordRes
  lists to act as allow/deny selectors. In the regular case won't need
  to specify anything or perhaps only the PLOAD4 list.

ENH: atomic creation of Nastran files

FIX: inconsistent Nastran surface output format

- use FREE format by default. Previously had an odd mix of SHORT
  format when created without options and LONG format (as default)
  when created with format options.