Bugfix: Provide const TensorIndicesHelper<>::extract variant
Bugfix: Provide const TensorIndicesHelper<>::extract variant

It is necessary to also deal with const references...

JaeryunYim/dealii,Arezou-gh/dealii,nicolacavallini/dealii,ESeNonFossiIo/dealii,JaeryunYim/dealii,kalj/dealii,sairajat/dealii,gpitton/dealii,angelrca/dealii,shakirbsm/dealii,johntfoster/dealii,kalj/dealii,shakirbsm/dealii,naliboff/dealii,shakirbsm/dealii,johntfoster/dealii,danshapero/dealii,johntfoster/dealii,andreamola/dealii,naliboff/dealii,ibkim11/dealii,shakirbsm/dealii,angelrca/dealii,YongYang86/dealii,johntfoster/dealii,pesser/dealii,adamkosik/dealii,YongYang86/dealii,ibkim11/dealii,maieneuro/dealii,angelrca/dealii,pesser/dealii,Arezou-gh/dealii,danshapero/dealii,JaeryunYim/dealii,EGP-CIG-REU/dealii,maieneuro/dealii,sairajat/dealii,spco/dealii,jperryhouts/dealii,andreamola/dealii,pesser/dealii,sairajat/dealii,ESeNonFossiIo/dealii,YongYang86/dealii,johntfoster/dealii,sairajat/dealii,naliboff/dealii,pesser/dealii,gpitton/dealii,ibkim11/dealii,Arezou-gh/dealii,shakirbsm/dealii,Arezou-gh/dealii,sairajat/dealii,johntfoster/dealii,EGP-CIG-REU/dealii,maieneuro/dealii,spco/dealii,danshapero/dealii,spco/dealii,jperryhouts/dealii,kalj/dealii,naliboff/dealii,danshapero/dealii,JaeryunYim/dealii,jperryhouts/dealii,kalj/dealii,andreamola/dealii,jperryhouts/dealii,maieneuro/dealii,YongYang86/dealii,adamkosik/dealii,nicolacavallini/dealii,Arezou-gh/dealii,danshapero/dealii,angelrca/dealii,spco/dealii,JaeryunYim/dealii,danshapero/dealii,danshapero/dealii,gpitton/dealii,nicolacavallini/dealii,pesser/dealii,nicolacavallini/dealii,andreamola/dealii,johntfoster/dealii,nicolacavallini/dealii,adamkosik/dealii,shakirbsm/dealii,YongYang86/dealii,YongYang86/dealii,pesser/dealii,kalj/dealii,ESeNonFossiIo/dealii,JaeryunYim/dealii,ibkim11/dealii,naliboff/dealii,gpitton/dealii,ESeNonFossiIo/dealii,YongYang86/dealii,EGP-CIG-REU/dealii,jperryhouts/dealii,EGP-CIG-REU/dealii,adamkosik/dealii,ibkim11/dealii,kalj/dealii,pesser/dealii,Arezou-gh/dealii,gpitton/dealii,angelrca/dealii,ESeNonFossiIo/dealii,spco/dealii,shakirbsm/dealii,angelrca/dealii,angelrca/dealii,ESeNonFossiIo/dealii,kalj/dealii,ESeNonFossiIo/dealii,EGP-CIG-REU/dealii,maieneuro/dealii,EGP-CIG-REU/dealii,jperryhouts/dealii,EGP-CIG-REU/dealii,maieneuro/dealii,gpitton/dealii,nicolacavallini/dealii,gpitton/dealii,JaeryunYim/dealii,spco/dealii,Arezou-gh/dealii,naliboff/dealii,spco/dealii,sairajat/dealii,andreamola/dealii,ibkim11/dealii,nicolacavallini/dealii,adamkosik/dealii,maieneuro/dealii,adamkosik/dealii,andreamola/dealii,adamkosik/dealii,jperryhouts/dealii,sairajat/dealii,andreamola/dealii,naliboff/dealii,ibkim11/dealii