Extended the .atom(s), .residue(s), .bond(s) and all other

indexing functions so that you can pass in an existing view or views as
the key. This lets you look up views in a container by other views, e.g.
`mols.bond(mols.atoms()[0], mols.atoms()[1])` would return the bond
between the first two atoms in the container `mols`.

Also added an `error_on_missing` flag to the `atoms`, `residues`, `bonds` etc
functions, so that you get a `KeyError` exception if there is no match,
and `error_on_missing` is `True`.

For example, `mols.atoms("element C", error_on_missing=True)` would raise
an exception if there are no carbon atoms in this container.

This is default `False` to keep existing behaviour, but we would recommend
setting this to `True` and would like to change the default in the future.

doc/source/changelog.rst

@@ -32,6 +32,19 @@ organisation on `GitHub <https://github.com/openbiosim/sire>`__.
   (default True) to rotate velocities as well as coordinates when usign
   a cursor to rotate molecule views. This fixes issue #103.
 
+* Extended the ``.atom(s)``, ``.residue(s)``, ``.bond(s)`` and all other
+  indexing functions so that you can pass in an existing view or views as
+  the key. This lets you look up views in a container by other views, e.g.
+  ``mols.bond(mols.atoms()[0], mols.atoms()[1])`` would return the bond
+  between the first two atoms in the container ``mols``. Also added
+  a ``error_on_missing`` flag to the ``atoms``, ``residues``, ``bonds`` etc
+  functions, so that you get a ``KeyError`` exception if there is no match,
+  and ``error_on_missing`` is ``True``. For example,
+  ``mols.atoms("element C", error_on_missing=True)`` would raise an exception
+  if there are no carbon atoms in this container. This is default ``False``
+  to keep existing behaviour, but we would recommend setting this to ``True``
+  and would like to change the default in the future.
+
 * Please add an item to this changelog when you create your PR
 
 `2023.4.1 <https://github.com/openbiosim/sire/compare/2023.4.0...2023.4.1>`__ - October 2023