Update requirements for building docs

- Mock requirements that are not needed using 'autodoc_mock_imports'
- Add necessary requirements to requirements.txt
- These changes should mean the docs can be built without first
  installing gprMax

docs/requirements.txt

@@ -1,5 +1,9 @@
 cython>=0.20
 numpy>=1.9
+colorama
+reframe-hpc
+scipy
+typing_extensions
 
 # Note: beware of https://github.com/readthedocs/sphinx_rtd_theme/issues/1115
 docutils==0.17.1

docs/source/conf.py

@@ -29,6 +29,16 @@ extensions = [
     "sphinx.ext.autosummary",
 ]
 
+autodoc_mock_imports = [
+    "h5py",
+    "humanize",
+    "mpi4py_fft",
+    "mpi4py",
+    "psutil",
+    "terminaltables",
+    "tqdm",
+]
+
 # Figure numbering
 numfig = True