Added spellcheck utility to documentation build

Uses the sphinxcontrib-spelling extension and requires PyEnchant and
aspell-en to be installed.

Adds the optional make target 'spelling' which produces the file
spelling/output.txt. It contains all words it detects.

Due to the large number of false positives, words in
utils/sphinx-config/false_positives.txt are ignored.

doc/.gitignore

@@ -1,4 +1,5 @@
 /html
+/spelling
 /LAMMPS.epub
 /LAMMPS.mobi
 /Manual.pdf

doc/Makefile

@@ -22,7 +22,7 @@ endif
 SOURCES=$(wildcard src/*.txt)
 OBJECTS=$(SOURCES:src/%.txt=$(RSTDIR)/%.rst)
 
-.PHONY: help clean-all clean epub html pdf old venv
+.PHONY: help clean-all clean epub html pdf old venv spelling
 
 # ------------------------------------------
 
@@ -44,6 +44,10 @@ clean-all:
 
 clean:
 	rm -rf $(RSTDIR) html
+	rm -rf spelling
+
+clean-spelling:
+	rm -rf spelling
 
 html: $(OBJECTS)
 	@(\
@@ -64,6 +68,17 @@ html: $(OBJECTS)
 	@rm -rf html/USER/*/*.[sg]*
 	@echo "Build finished. The HTML pages are in doc/html."
 
+spelling: $(OBJECTS) utils/sphinx-config/false_positives.txt
+	@(\
+		. $(VENV)/bin/activate ;\
+		pip install sphinxcontrib-spelling ;\
+		cp -r src/* $(RSTDIR)/ ;\
+        cp utils/sphinx-config/false_positives.txt $(RSTDIR)/ ;\
+		sphinx-build -b spelling -c utils/sphinx-config -d $(BUILDDIR)/doctrees $(RSTDIR) spelling ;\
+		deactivate ;\
+	)
+	@echo "Spell check finished."
+
 epub: $(OBJECTS)
 	@mkdir -p epub
 	@rm -f LAMMPS.epub

doc/utils/sphinx-config/conf.py

@@ -299,4 +299,11 @@ more information about the code and its uses.
 
 epub_author = 'The LAMMPS Developers'
 
+# configure spelling extension if present
+import importlib.util
+spelling_spec = importlib.util.find_spec("sphinxcontrib.spelling")
+if spelling_spec:
+    extensions.append('sphinxcontrib.spelling')
 
+    spelling_lang='en_US'
+    spelling_word_list_filename='false_positives.txt'