Remove unused variables reported by compiler

When compiling SELF with warning flags enabled ( `cmake -DCMAKE_BUILD_TYPE=debug`), there are a number of subroutines that are identified as having declared but unused variables. In most cases these ought to be removed. In some cases, this is intentional; e.g. for our template routines that perform no action unless overridden by the end user.

We need some help cleaning this up.

**Anyone who wants to tackle this will be given a free account on [Fluid Numerics Galapagos cluster](https://galapagos.fluidnumerics.com)**.