auto data/define swith; no except. in dtor; addVar for scalars; open+format in ncFile ctor

Hi,

76a61b9: With this patch, when working on classic files, the data/define mode swithing is done automatically by the library (see example in cxx4/test_classic.cpp).

0d1698d: Avoiding undefined behaviour when an exception is being thrown while handling an exception

fb1f81a: addVar() version for scalar variables

920860a: opening files with non-default format in NcFile ctor

HTH,
Sylwester