syncIO 'random access'

[cwsmith]

The functionality is already implemented but in two different ways for the old posix code and for the syncio code.

• The syncio code relies on the lookup table stored in the master header to get the the right address of a field in the file. Currently, this lookup table is activat only when the phastaIO lib is used in parallel with global access from all ranks but we are clearly missing the interface for a single rank to access it independently from the other ranks (back to "serial processing" in the wish list)
• The old posix code jumps from one field header to the next one (the size of the data block associated with each field is stored in its header between the < > signs) until it finds the target field. The current posix implementation cannot work with the current syncio files simply because it does not how to handle the master header (and jump over it). If the posix code starts a new search from its current location and has not found the target when it reaches the bottom of the files, it rewinds and start over until it finds the target (or reaches the starting point in the file and declare in this case that the field is missing).