Add mypy Action for Type Checking

[AndrewPlayer3]

No description provided.

[jtherrmann]

Turns out mypy wasn't flagging most of the errors because there's no __init__.py, I opened #113 to fix that.

[forrestfwilliams]

burst2safe S1_136231_IW2_20200604T022312_VV_7C85-BURST S1_136232_IW2_20200604T022315_VV_7C85-BURST

Is failing with the exception:

Traceback (most recent call last):
  File "/Users/ffwilliams2/miniforge3/envs/burst2safe/bin/burst2safe", line 8, in <module>
    sys.exit(main())
             ^^^^^^
  File "/Users/ffwilliams2/Repositories/burst2safe/src/burst2safe/burst2safe.py", line 108, in main
    burst2safe(
  File "/Users/ffwilliams2/Repositories/burst2safe/src/burst2safe/burst2safe.py", line 72, in burst2safe
    safe_path = safe.create_safe()
                ^^^^^^^^^^^^^^^^^^
  File "/Users/ffwilliams2/Repositories/burst2safe/src/burst2safe/safe.py", line 438, in create_safe
    self.create_safe_components()
  File "/Users/ffwilliams2/Repositories/burst2safe/src/burst2safe/safe.py", line 304, in create_safe_components
    swath.write()
  File "/Users/ffwilliams2/Repositories/burst2safe/src/burst2safe/swath.py", line 155, in write
    assert self.measurement.data_mean is not None
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError