Remove MPPI for readthedocs builds

.readthedocs.yaml

@@ -14,7 +14,7 @@ build:
       - python -m pip install sphinx_rtd_theme
 
 conda:
-  environment: conda_env.yml
+  environment: conda_env_no_mpi.yml
 
 python:
   install:

conda_env_no_mpi.yml

@@ -0,0 +1,26 @@
+# This file may be used to create a conda environment using:
+# $ conda env create -f <this file>
+
+name: gprMax-devel
+dependencies:
+- python>3.7
+- colorama
+- cython
+- h5py
+- jinja2
+- jupyter
+- matplotlib
+- numpy
+- pip
+- psutil
+- scipy
+
+- pip:
+  - humanize
+#  - mpi4py
+  - numpy-stl
+#  - pycuda
+#  - pyopencl
+  - terminaltables
+  - tqdm
+  - git+https://github.com/craig-warren/PyEVTK.git