Benchmarking (#187)

* Add make benchmarks

* Various tweaks related to the benchmarks.

* In Makefile:
* target 'all' now builds the library, all tests and examples
* the old 'all' target is n

Benchmarking (#187)

* Add make benchmarks

* Various tweaks related to the benchmarks.

* In Makefile:
* target 'all' now builds the library, all tests and examples
* the old 'all' target is now called 'par'
* the target 'examples' will build also the MFEM and PETSc examples if
the respective library is available.

In the benchmarks/ directory:
* remove 'config.sh'
* cleanup unused stuff from 'benchmark.sh'.

* Fix postprocess scripts, convert to Python 3

* Small update in README.md

* Set benchmark cg its max, update gitignore

* Minor makefile fix

* In Makefile, add 'par' to the list of phony targets.

* In benchmarks/postprocess-table.py, sort the table by backend first.

* Small update in examples/petsc/Makefile - add a comment that
PETSC_ARCH can be undefined/empty, e.g. when using PETSc installed
through Spack.

* In Makefile, update the benchmarking targets:
* add separate targets for individual tests: `bench-petsc-bp1`,
`bench-petsc-bp3`, etc
* `make benchmarks` runs all defined benchmarks.

Update README.md to reflect the above changes.

show more ...