Add scripts to simplify offline builds of docs and CMake

tools/offline/scripts/init_pip_cache.sh

@@ -0,0 +1,22 @@
+#!/bin/bash
+
+if [ -z "${LOGGING_DIR}" ]
+then
+    echo "Must set LOGGING_DIR environment variable"
+    exit 1
+fi
+
+if [ -z "${PIP_CACHE_DIR}" ]
+then
+    echo "Must set PIP_CACHE_DIR environment variable"
+    exit 1
+fi
+
+set -x
+
+mkdir -p $PIP_CACHE_DIR
+
+# download packages that might be needed
+cd $PIP_CACHE_DIR
+pip3 download pip setuptools wheel
+pip3 download -r $LAMMPS_DIR/doc/utils/requirements.txt