BUG: io/netcdf: Call mmap only once to prevent too many open files
TST: io/netcdf: remove unnecessary use of test generators

Test generators are best used only for iterating over parameter sets,
and otherwise best avoided.
BUG: io/netcdf: clean up code a bit
Merge pull request #3429 from pr-3429

BUG: io/netcdf: use only a single mmap in netcdf

Using a separate mmap for each access of each varible can cause running
out of file descriptors on some platforms.  Address this by using a
single mmap covering the whole file.