Rewrite the documentation of the BlockIndices class.
Rewrite the documentation of the BlockIndices class.

The existing documentation described mostly *how* things are done, but not
what the class actually *represents*.

spco/dealii,jperryhouts/dealii,jperryhouts/dealii,spco/dealii,kalj/dealii,JaeryunYim/dealii,naliboff/dealii,sairajat/dealii,gpitton/dealii,angelrca/dealii,sairajat/dealii,shakirbsm/dealii,angelrca/dealii,gpitton/dealii,jperryhouts/dealii,ESeNonFossiIo/dealii,nicolacavallini/dealii,maieneuro/dealii,JaeryunYim/dealii,nicolacavallini/dealii,maieneuro/dealii,shakirbsm/dealii,EGP-CIG-REU/dealii,danshapero/dealii,JaeryunYim/dealii,maieneuro/dealii,jperryhouts/dealii,angelrca/dealii,spco/dealii,EGP-CIG-REU/dealii,andreamola/dealii,angelrca/dealii,maieneuro/dealii,danshapero/dealii,shakirbsm/dealii,jperryhouts/dealii,maieneuro/dealii,ESeNonFossiIo/dealii,danshapero/dealii,spco/dealii,pesser/dealii,ESeNonFossiIo/dealii,shakirbsm/dealii,JaeryunYim/dealii,pesser/dealii,ESeNonFossiIo/dealii,andreamola/dealii,ESeNonFossiIo/dealii,naliboff/dealii,kalj/dealii,andreamola/dealii,kalj/dealii,shakirbsm/dealii,maieneuro/dealii,danshapero/dealii,sairajat/dealii,gpitton/dealii,naliboff/dealii,sairajat/dealii,jperryhouts/dealii,sairajat/dealii,JaeryunYim/dealii,gpitton/dealii,danshapero/dealii,pesser/dealii,gpitton/dealii,spco/dealii,danshapero/dealii,angelrca/dealii,pesser/dealii,nicolacavallini/dealii,jperryhouts/dealii,EGP-CIG-REU/dealii,spco/dealii,EGP-CIG-REU/dealii,pesser/dealii,gpitton/dealii,JaeryunYim/dealii,angelrca/dealii,shakirbsm/dealii,JaeryunYim/dealii,andreamola/dealii,pesser/dealii,naliboff/dealii,naliboff/dealii,EGP-CIG-REU/dealii,spco/dealii,naliboff/dealii,ESeNonFossiIo/dealii,maieneuro/dealii,kalj/dealii,kalj/dealii,sairajat/dealii,nicolacavallini/dealii,nicolacavallini/dealii,ESeNonFossiIo/dealii,EGP-CIG-REU/dealii,sairajat/dealii,angelrca/dealii,nicolacavallini/dealii,shakirbsm/dealii,nicolacavallini/dealii,kalj/dealii,andreamola/dealii,EGP-CIG-REU/dealii,pesser/dealii,andreamola/dealii,kalj/dealii,gpitton/dealii,andreamola/dealii,naliboff/dealii,danshapero/dealii