Add a SConsPackage base class (#4936)

* Add a SConsPackage base class

* Make Matlab extendable

* Most dependencies are actually required

* Cantera requires older version of fmt

var/spack/repos/builtin/packages/matlab/package.py

@@ -66,6 +66,8 @@ class Matlab(Package):
     license_vars     = ['LM_LICENSE_FILE']
     license_url      = 'https://www.mathworks.com/help/install/index.html'
 
+    extendable = True
+
     def url_for_version(self, version):
         return "file://{0}/matlab_{1}_glnxa64.zip".format(os.getcwd(), version)