Remove stateful updates (#192)

* Rewrite direct connection application to accumulate batch results

* Update inverse projection connection application

* Update dense resampling matrix routine

* Remove unused loop variable

* Fix direct connection application; pad with zeros if no batch data

* Update simple-dg example

* Update flatten routine for DOFArrays

* Special case for dof arrays with one group (avoids a copy)

* Remove deprecated methods/operations

* Clarify comment

Co-authored-by: Alex Fikl <alexfikl@gmail.com>

* Sharpen conditional for zero-copy flatten

Co-authored-by: Alex Fikl <alexfikl@gmail.com>
Co-authored-by: Andreas Kloeckner <inform@tiker.net>