RecordType and DataSet support in C.

[rainwoodman]

These changes will allow us to use bigfile to do journals (e.g. record blackhole details per step).

1. RecordType = [ ( column name, dtype ) ]

   big_record_set(rt, void* record_buf, icol, const void * value)
   big_record_get(rt, const void* record_buf, icol, void * value)

2. BigDataset(RecordType, File).
   big_dataset_read(bd, offset, size, buf), big_dataset_write(bd, offset, size, buf)
   Use BigArray to view the buf from the correct strides, and big_block_read / big_block_write to do the writing on each block in the data set.
   big_dataset_read_mpi, big_dataset_write_mpi,
   same as above. may need to plumb some flags.
   big_dataset_grow(bd, size)
   big_dataset_grow_mpi(bd, size)

Concern 1. may need to open and close 2xNcolumn physical files per read/write.
Is it fast enough for a PM step? Probably fine.

Concern 2. if some blocks in the record type exists, some doesn't exist? Perhaps need to create and grow on the fly.