Clean up the handling of chunking parameters (the -c flag) in nccopy.

[DennisHeimbigner]

• Fixes Nccopy ignores chunk spec when input is netcdf-4 contiguous #725
• Fixes Seeing compile warnings in bigmeta.c #1079

This PR primarily addresses Issue #725.
After a long discussion, I implemented the rules at the end of that issue.
They are documented in nccopy.1.

Additionally, I added a new, per-variable, -c flag that allows
for the direct setting of the chunking parameters for a variable.
The form is
-c var:c1,c2,...ck
where var is the name of the variable (possibly a fully qualified name)
and the ci are the chunksizes for that variable. It must be the case
that the rank of the variable is k. If the new form is used as well
as the old form, then the new form overrides the old form for the
specified variable. Note that multiple occurrences of the new form
-c flag may be specified.

Misc. Other fixes

1. Added -M option to nccopy to specify the minimum
   allowable chunksize.
2. Removed the unused variables from bigmeta.c
   (Issue Seeing compile warnings in bigmeta.c #1079)
3. Fixed failure of nc_test4/tst_filter.sh by using the new -M
   flag (Netcdf cmake #1) to allow filter test on a small chunk size.

[WardF]

Reviewing now then will merge. Thanks @DennisHeimbigner !

[WardF]

Seeing a failure on Windows in ncdump/tst_nccopy4.sh. Specifically, in the 'test that nccopy -c can chunk and unchunk files' I'm gettin ga 'Variable not found' error. This is different from the error sometimes seen on master currently.

[WardF]

Specifically, it is failing on nccopy -c / tmp-chunked.nc tmp-unchunked.nc

[WardF]

Simple fix, changing -c / to -c //, appears to fix the problem. @DennisHeimbigner any issues with this approach?

[DennisHeimbigner]

nccopy -c / tmp-chunked.nc tmp-unchunked.nc
Interesting. It seemed to be ok with my version of visual studio.
If the string that nccopy gets internally is '/' for '//' then ok. If it is getting
'//', then not ok.
Will '/' work? (that is use quotes on the command line).