Fix to allow cubed to work without diagnostics

[tomwhite]

Fix for the following bug:

% pip install 'git+https://github.com/cubed-dev/cubed.git' 
% python
>>> import cubed
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/tom/workspace/cubed/cubed/__init__.py", line 16, in <module>
    import cubed.random  # noqa: F401
    ^^^^^^^^^^^^^^^^^^^
  File "/Users/tom/workspace/cubed/cubed/random.py", line 7, in <module>
    from cubed.core.ops import map_blocks
  File "/Users/tom/workspace/cubed/cubed/core/__init__.py", line 2, in <module>
    from .array import CoreArray, compute, gensym, measure_reserved_mem, visualize
  File "/Users/tom/workspace/cubed/cubed/core/array.py", line 15, in <module>
    from .plan import FinalizedPlan, arrays_to_plan
  File "/Users/tom/workspace/cubed/cubed/core/plan.py", line 16, in <module>
    from cubed.diagnostics.colors import APRICOT, LAVENDER, RED
  File "/Users/tom/workspace/cubed/cubed/diagnostics/__init__.py", line 1, in <module>
    from .rich import RichProgressBar as ProgressBar
  File "/Users/tom/workspace/cubed/cubed/diagnostics/rich.py", line 6, in <module>
    from rich.console import RenderableType
ModuleNotFoundError: No module named 'rich'