Support fill value conversion in vlen types

[mjwoods]

This PR adds support for fill values in vlen types when na.mode=5 is set for var.put.nc or var.get.nc.

The _FillValue attribute is required to have the same vlen type as the variable, and it must contain only a single vlen element. The first basetype value in the vlen element is used as the fill value. This is the approach used in the netcdf library, as described in
Unidata/netcdf-c#1011 (comment) .

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.09 🎉

Comparison is base (f26d33a) 80.37% compared to head (c869f37) 80.46%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #120      +/-   ##
==========================================
+ Coverage   80.37%   80.46%   +0.09%     
==========================================
  Files          13       13              
  Lines        5777     5799      +22     
==========================================
+ Hits         4643     4666      +23     
+ Misses       1134     1133       -1     

Impacted Files	Coverage Δ
src/convert.c	80.10% <100.00%> (+0.11%)	⬆️

... and 1 file with indirect coverage changes

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[mjwoods]

Tests of nested vlen types are only successful for NetCDF versions 4.9.0 and later, which include PR Unidata/netcdf-c#2179.

To ensure that tests succeed on all supported versions of NetCDF (from 4.1.3 upwards), we avoid testing nested vlen types on NetCDF versions less than 4.9.0 (see d7b17f5).

[mjwoods]

Tests of vlen with fill value (other than nested vlen types) were successful with NetCDF versions 4.1.3, 4.8.0, 4.9.0 and 4.9.2.

The tests also worked successfully with gctorture (netcdf-4.1.3, R-4.3.0, RedHat 8), using the following command:
R CMD check --no-examples --use-gct RNetCDF_2.7-1.tar.gz

[mjwoods]

Running the tests with both --use-gct and --use-valgrind found no memory leaks (with netcdf-4.9.2, R-4.3.0, RedHat 8).