enable compact storage as an additional storage option for netCDF-4 vars

[edwardhartnett]

HDF5 supports a "compact" storage option. It helps a lot with parallel I/O performance, but may only be used for small variables (< 64 MB).

See #1554 for prototype implementation by @gsjaardema, who suggested this feature.

This can be accomplished without API changes. Currently we have two constants for nc_def_var_chunking():

/** In HDF5 files you can set storage for each variable to be either
 * contiguous or chunked, with nc_def_var_chunking().  This define is
 * used there. */
/**@{*/
#define NC_CHUNKED    0
#define NC_CONTIGUOUS 1
/**@}*/

By adding NC_COMPACT as a third constant, we can enable compact storage for users.

This feature is backward compatible to HDF5 1.8.x.