fix hard-coded field limit in phastaIO

[cwsmith]

• 1. Copy the test phIOwriteReadZeroSz.cc to create a new test named phIOwriteFields.cc
  (cp phIOwriteReadZeroSz.cc phIOwriteFields.cc).
  phIOwriteReadZeroSz.cc reads an integer from the command line specifying the number of syncio files to create.
• 2. Add the test to testing.cmake as is done for phIOwriteReadZeroSz.cc
• 3. Hard code the number of syncio files to create by setting nfiles to one on line 31.
• 4. Lines 44 and 45 write a header and its associated data block
  Add a for loop that starts above line 44 and ends after line 45. The loop should run a user specified (see line 31) number of times; each time it writes a header and a data block.
• 5. At the top of the loop create a string that specifies the header and data block name. This name should be unique; use the loop number in the string. Replace the phrase argument on lines 44 (the call to phio_writeheader) and 45 (the call to phio_writedatablock) with the created string. Note, these functions expect the string to be a const char*.
• 6. Repeat the loop creation done in steps four and five for the calls to phio_readheader and phio_readdatablock on lines 56 and 58. At run time this will read back the headers and data blocks that were created by the loop in step four.
• 7. manually run the test with an increasing field count until it breaks (likely in syncio)
• 8. debug the code to find the problem
• 9. fix the problem
• 10. re-run the test