Add codespell support (config, workflow to detect/not fix) and make it fix some typos

.codespellrc

@@ -0,0 +1,5 @@
+[codespell]
+# Ref: https://github.com/codespell-project/codespell#using-a-config-file
+skip = .git*,.codespellrc
+check-hidden = true
+ignore-words-list = datas,acn

.github/workflows/codespell.yml

@@ -0,0 +1,25 @@
+# Codespell configuration is within .codespellrc
+---
+name: Codespell
+
+on:
+  push:
+    branches: [master]
+  pull_request:
+    branches: [master]
+
+permissions:
+  contents: read
+
+jobs:
+  codespell:
+    name: Check for spelling errors
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+      - name: Annotate locations with typos
+        uses: codespell-project/codespell-problem-matcher@v1
+      - name: Codespell
+        uses: codespell-project/actions-codespell@v2

docs/spherical-video-rfc.md

@@ -72,7 +72,7 @@ Spherical video metadata is stored in a uniquely-identified *moov.trak.uuid* box
 
 #### Stereo Mode
 
-[SEI Frame Packing Arragement](http://www.itu.int/ITU-T/recommendations/rec.aspx?rec=10635) and the [StereoMode](http://www.matroska.org/technical/specs/index.html#StereoMode) tag for Matroska/WebM video files can be used to describe the left/right frame layout. To include non-h264 MPEG-4 files an additional StereoMode tag will override the native stereo configuration. The supported StereoMode values are shown below with the corresponding native values.
+[SEI Frame Packing Arrangement](http://www.itu.int/ITU-T/recommendations/rec.aspx?rec=10635) and the [StereoMode](http://www.matroska.org/technical/specs/index.html#StereoMode) tag for Matroska/WebM video files can be used to describe the left/right frame layout. To include non-h264 MPEG-4 files an additional StereoMode tag will override the native stereo configuration. The supported StereoMode values are shown below with the corresponding native values.
 
 | **Name** | **Description** |Equivalent MKV Value | Equivalent h264 SEI FPI |
 |----------|-----------------|---------------------|-------------------------|

spatialmedia/mpeg/box.py

@@ -155,7 +155,7 @@ def index_copy(in_fh, out_fh, box, mode, mode_length, delta=0):
       out_fh: file handle, destination for index file.
       box: box, stco/co64 box to copy.
       mode: string, bit packing mode for index entries.
-      mode_length: int, number of bytes for index entires.
+      mode_length: int, number of bytes for index entries.
       delta: int, offset change for index entries.
     """
     fh = in_fh