Align str/repr of core types

[pp-mo]

Since providing a 'useful' AttrvalsDict.str(), find that this is not the repr(), which instead looks like <ncdata._core.AttrvalsDict object at 0x...>
This is consistent for most core objects, e.g. <ncdata._core.NcVariable object at 0x...>
But a NameMap object doesn't do that, e.g.

>>> ds
<ncdata._core.NcData object at 0x0000024F7ADFC2F0>
>>> ds.variables['v']
<ncdata._core.NcVariable object at 0x0000024F7B8824E0>

but ..

>>> ds.groups
{}
>>> ds.variables
{'v': <ncdata._core.NcVariable object at 0x0000024F7B8824E0>}
>>>```