Document feature eigenvalues_only

SWITCHING_TO_NEW_INTERFACE.md

@@ -124,6 +124,8 @@ the user application):
 
    "debug"    can be one of { 0 | 1 }, will give more information case of an error if set to 1
 
+   "eigenvalues_only" can be one of { 0 | 1 }, if set to 1 solve will only compute the eigenvalues
+
 
 6. do the desired task with the *ELPA* library, which could be
    a) e%solve                         ! solve EV problem with solver as set by "set" method;

USERS_GUIDE.md

@@ -59,6 +59,7 @@ Using *ELPA* with the latest API is done in the following steps
        of the ELPA library has been build with GPU support enables
      - "timings" can be either 0 or 1, switches time measurements off or on
      - "debug" can be either 0 or 1, switches detailed debug messages off/on
+     - "eigenvalues_only" can be either 0 or 1. In case of 1 only compute the eigenvalues
 
 - call ELPA-type function solve or others