Prepare for 0.7.0 release (#604)

This commit prepares for the pending 0.7.0 release by moving the release
notes to a dedicated directory for the release. This will likely be the
last minor release for qiskit-ignis as it is now deprecated and we'll
only be making critical bug and compatibility fixes moving forward.

releasenotes/notes/0.7/analytical-syndrome-graph-1cbc0a900c987ad8.yaml

@@ -0,0 +1,8 @@
+---
+features:
+ - |
+ Added the option for the fast analytical generation of syndrome graphs.
+ The :class:`.RepetitionCode` now has a new bool argument ``brute``, which
+ allows to still use the brute force method.
+ Helper class :class:`.RepetitionCodeSyndromeGenerator` added to
+ facilitate this.

releasenotes/notes/0.7/optional-resets-and-delays-2cd301f1257b3962.yaml

@@ -0,0 +1,23 @@
+
+features:
+ - |
+ The :class:`~qiskit.ignis.verification.RepetitionCode` now has keyword
+ arguments ``resets`` and ``delay``. The former determines whether reset
+ gates are inserted after measurement. The latter allows a time (in dt) to
+ be specificed for a delay after each measurement (and reset, if applicable).
+ 
+ The :meth:`~qiskit.ignis.verification.RepitionCode.syndrome_measurement` method of
+ :class:`~qiskit.ignis.verification.RepetitionCode` now has keyword
+ arguments ``final`` and ``delay``. The former determines whether to add reset gates according
+ to the global ``resets``, or to overwrite it with appropriate behavior for the
+ final round of syndrome measurements. The latter allows a time (in dt) to be specificed 
+ for a delay after each measurement (and reset, if applicable).
+upgrade:
+ - |
+ The keyword argument ``reset`` has been removed from the
+ the :meth:`~qiskit.ignis.verification.RepitionCode.syndrome_measurement`
+ method of :class:`~qiskit.ignis.verification.RepetitionCode`. This is
+ replaced by the global ``resets`` keyword argument for the class as well as
+ the keyword argument ``final`` for ``syndrome_measurement``. In cases where
+ one would previously add the final measurement round using ``reset=False``
+ to avoid the final reset gates, one should now use ``final=True``.

releasenotes/notes/0.7/xbasis-encoding-e9d008b027b5d7d9.yaml

@@ -0,0 +1,6 @@
+---
+features:
+ - |
+ The :class:`.RepetitionCode` class now supports encoding with x basis
+ states. This can be used by setting the ``xbasis`` keyword argument when
+ constructing a :class:`.RepetitionCode` object.