annoying constraint on the filename of instrumentation data

[vext01]

There is an annoying constraint on the filename of instrumentation data. This Makefile snippet from the upcoming v0.2 artefact explains:

# Note that this file, and the directory containing the actual instrumentation
# data, has been renamed. To find the instrumentation data, the plotting script
# expects to be able to take the results file name, remove the
# `_results.json.bz2` suffix and append `_instr_data`. This will be fixed in
# later versions.
EXAMPLE_INSTR_DATA=warmup/linux3_e3_1240/with_instrumentation/instr_results_0_8_linux3_e3_1240_results.json.bz2

I think we should have a switch instead.