added patch to switch to fwbw_derivative

installation/patch/README.md

@@ -0,0 +1,17 @@
+# DAMASK patching
+
+This folder contains patches that modify the functionality of the current version of DAMASK prior to the corresponding inclusion in the official release.
+
+## Usage
+
+```bash
+cd DAMASK_ROOT
+patch -p1 installation/patch/nameOfPatch
+```
+
+## Available patches
+
+  * **fwbw_derivative** switches the default spatial derivative from continuous to forward/backward difference.  
+    This generally reduces spurious oscillations in the result as the spatial accuracy of the derivative is then compatible with the underlying solution grid.
+  * **petsc3.7** adapts to API changes introduced between PETSc 3.6.x and 3.7.x for setting PETSc options.  
+    Use this patch if your system runs PETSc 3.7.x.

installation/patch/fwbw_derivative

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+diff --git a/code/numerics.f90 b/code/numerics.f90
+index 24bd190..c968c70 100644
+--- a/code/numerics.f90
++++ b/code/numerics.f90
+@@ -110,7 +110,7 @@ module numerics
+    fftw_plan_mode             = 'FFTW_PATIENT'                                                      !< reads the planing-rigor flag, see manual on www.fftw.org, Default FFTW_PATIENT: use patient planner flag
+  character(len=64), protected, public :: & 
+    spectral_solver            = 'basicpetsc'  , &                                                   !< spectral solution method 
+-   spectral_derivative        = 'continuous'                                                        !< spectral spatial derivative method
++   spectral_derivative        = 'fwbw_difference'                                                   !< spectral spatial derivative method
+  character(len=1024), protected, public :: &
+    petsc_defaultOptions       = '-mech_snes_type ngmres &
+                                 &-damage_snes_type ngmres &