Add periodic table with available radii

Author: Roberto Di Remigio

CHANGELOG.md

@@ -10,6 +10,9 @@
 API kills program execution if this is the case.
 - An API function to retrieve the areas/weights of the cavity finite elements.
 The values in the returned array are in Bohr^2. Addresses a feature request from @shofener (Issue #13)
+- The standalone executable `run_pcm` is now tested within the unit tests suite.
+The tests cover the cases where the cavity is given implicitly, explicitly or by substitution of
+radii on chosen atoms.
 
 ### Changed
 
@@ -25,6 +28,9 @@ the cavity.
 not use C++ exceptions.
 - `include`-s are now specified on a per-directory basis (see programmers' manual
 for a more detailed explanation)
+- Default types for template paramters `DerivativeTraits`,  `IntegratorPolicy` and `ProfilePolicy`
+are now given for the Green's functions classes. This reduced the verbosity in instatiating
+these objects significantly.
 
 ### Known Issues