Add experimental support for fixed length string type to nczarr.

[DennisHeimbigner]

re: Issue #2465
re: Issue #2259

[Note: It also tangentially affects PR https://github.com//pull/2466 since this PR requires that PR to be merged before this one and actually includes that PR here.]

The primary issue to be addressed is to provide a way for user to
specify the size of the fixed length strings. This is handled by providing
the following new attributes special:

1. _nczarr_default_maxstrlen —
   This is an attribute of the root group. It specifies the default
   maximum string length for string types. If not specified, then
   it has the value of 64 characters.
2. _nczarr_maxstrlen &mdash
   This is a per-variable attribute. It specifies the maximum
   string length for the string type associated with the variable.
   If not specified, then it is assigned the value of
   _nczarr_default_maxstrlen.

This PR also requires some hacking to handle the existing netcdf-c NC_CHAR
type, which does not exist in zarr. The goal was to choose numpy types for
both the netcdf-c NC_STRING type and the netcdf-c NC_CHAR type such that
if a pure zarr implementation read them, it would still work and an
NC_CHAR type would be handled by zarr as a string of length 1.

For writing variables and NCZarr attributes, the type mapping is as follows:

• "|S1" for NC_CHAR.
• ">S1" for NC_STRING && MAXSTRLEN==1
• ">Sn" for NC_STRING && MAXSTRLEN==n

Note that it is a bit of a hack to use endianness, but it should be ok since for
string/char, the endianness has no meaning.

For reading attributes with pure zarr (i.e. with no nczarr
atribute types defined), they will always be interpreted as of
type NC_CHAR.

Misc. Other Changes

1. Convert the nczarr special attributes and keys to be all lower case. So "_NCZARR_ATTR" now used "_nczarr_attr. Support back compatibility for the upper case names.
2. Cleanup my too-clever-by-half handling of scalars in libnczarr.

[DennisHeimbigner]

This PR introduces some memory leaks, so I am closing it in favor
of a more general PR.

[DennisHeimbigner]

Subsumed by PR #2492