computed quat thing doesn't print well

[jeanconn]

I think this is basically a repr issue but it is awkward for the user -

In [1]: from cheta import fetch

In [2]: fetch.Msid('quat_aoattqt', '2024:001', '2024:002')
Out[2]: ---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
File /fido.real/conda/envs/ska3-flight-2024.1rc4/lib/python3.11/site-packages/IPython/core/formatters.py:711, in PlainTextFormatter.__call__(self, obj)
    704 stream = StringIO()
    705 printer = pretty.RepresentationPrinter(stream, self.verbose,
    706     self.max_width, self.newline,
    707     max_seq_length=self.max_seq_length,
    708     singleton_pprinters=self.singleton_printers,
    709     type_pprinters=self.type_printers,
    710     deferred_pprinters=self.deferred_printers)
--> 711 printer.pretty(obj)
    712 printer.flush()
    713 return stream.getvalue()

File /fido.real/conda/envs/ska3-flight-2024.1rc4/lib/python3.11/site-packages/IPython/lib/pretty.py:411, in RepresentationPrinter.pretty(self, obj)
    408                         return meth(obj, self, cycle)
    409                 if cls is not object \
    410                         and callable(cls.__dict__.get('__repr__')):
--> 411                     return _repr_pprint(obj, self, cycle)
    413     return _default_pprint(obj, self, cycle)
    414 finally:

File /fido.real/conda/envs/ska3-flight-2024.1rc4/lib/python3.11/site-packages/IPython/lib/pretty.py:779, in _repr_pprint(obj, p, cycle)
    777 """A pprint that just redirects to the normal repr function."""
    778 # Find newlines and replace them with p.break_()
--> 779 output = repr(obj)
    780 lines = output.splitlines()
    781 with p.group():

File /fido.real/conda/envs/ska3-flight-2024.1rc4/lib/python3.11/site-packages/cheta/fetch.py:601, in MSID.__repr__(self)
    595 def __repr__(self):
    596     attrs = [self.__class__.__name__, self.MSID]
    597     for name, val in (
    598         ("start", self.datestart),
    599         ("stop", self.datestop),
    600         ("len", len(self)),
--> 601         ("dtype", self.dtype.name),
    602         ("unit", self.unit),
    603         ("stat", self.stat),
    604     ):
    605         if val is not None:
    606             attrs.append("{}={}".format(name, val))

File /fido.real/conda/envs/ska3-flight-2024.1rc4/lib/python3.11/site-packages/cheta/fetch.py:593, in MSID.dtype(self)
    591 @property
    592 def dtype(self):
--> 593     return self.vals.dtype

AttributeError: 'Quat' object has no attribute 'dtype'