Fix for scikit sparse dependency issue (#7)

* Remove scikit-sparse from requirements in setup.py
* Add requirements.in, specifying direct dependencies
* Add requirements.txt, compiled from requirements.in
* The base python version was also switched to 3.6, as it has been
tested more than 3.8.
* Add note in README on optional scikit-sparse dep.

README.md

@@ -53,6 +53,8 @@ your choice:
 
 `pip3 install --user --src /my/preferred/location --editable git+https://github.com/Deep-MI/Lapy.git#egg=lapy`
 
+Several functions, e.g. the Solver, require a sparse matrix decomposition, for which either the LU decomposition (from scipy sparse) or the faster Cholesky decomposition (from scikit-sparse cholmod) can be used. If the parameter flag use_cholmod is True, the code will try to import cholmod from the scikit-sparse package and will fall back to LU if the import fails. If you would like to use cholmod, you need to install scikit-sparse separately. It cannot be listed among LaPy's dependencies (e.g. in setup.py or requirements.txt) as that causes errors with pip. scikit-sparse requires numpy and scipy to be installed separately beforehand.
+
 ## References:
 
 If you use this software for a publication please cite both these papers:

requirements.in

@@ -0,0 +1,3 @@
+numpy==1.17.4
+scipy==1.3.1
+plotly==5.5.0

requirements.txt

@@ -1,22 +1,17 @@
 #
-# This file is autogenerated by pip-compile with python 3.8
+# This file is autogenerated by pip-compile with python 3.6
 # To update, run:
 #
 #    pip-compile
 #
-numpy==1.21.4
+numpy==1.17.4
     # via
-    #   LaPy (setup.py)
-    #   scikit-sparse
+    #   -r requirements.in
     #   scipy
-plotly==5.3.1
-    # via LaPy (setup.py)
-scikit-sparse==0.4.6
-    # via LaPy (setup.py)
-scipy==1.7.2
-    # via
-    #   LaPy (setup.py)
-    #   scikit-sparse
+plotly==5.5.0
+    # via -r requirements.in
+scipy==1.3.1
+    # via -r requirements.in
 six==1.16.0
     # via plotly
 tenacity==8.0.1

setup.py

@@ -49,5 +49,5 @@ def get_version(rel_path):
     # For an analysis of "install_requires" vs pip's requirements files see:
     # https://packaging.python.org/en/latest/requirements.html
 #    install_requires=['peppercorn'],  # Optional
-    install_requires=['numpy', 'scipy', 'plotly', 'scikit-sparse'],
+    install_requires=['numpy', 'scipy', 'plotly'],
 )