ENH: runtime HEATMAP/DXT cross-check log

* add a carefully crafted darshan log using latest `darshan`
`main` branch that contains "runtime" `HEATMAP` module
and DXT tracing, to facilitate the writing of carefully
designed tests for internal consistency of data structures
(and to facilitate rapid visual inspection as well)

* see the `README` for a very detailed description of how
the data was produced (this is perhaps a bit too detailed,
though I'm mostly doing this so that more folks can generate
carefully-crafted testing data using pure Python without
waiting for me for example)

darshan_logs/runtime_and_dxt_heatmaps_diagonal_write_only/README.md

@@ -0,0 +1,58 @@
+This is an example darshan log that contains both runtime
+heatmap module data and DXT tracing, to facilitate both
+visual and robust numerical comparison of the two approaches
+of generating heatmaps in the new Python summary report.
+
+The log follows the following "graphical" IO pattern
+of writing only a single byte of data per rank, for each
+of 32 ranks, on staggered/exaggerated delays:
+
+Time Bins -->
+Rank 
+|
+|
+V
+      31        x
+      30       x
+      29      x
+      28     x
+      ..
+      3    x
+      2   x
+      1  x
+      0 x
+
+The example application code used to generate the log
+is available here:
+https://github.com/tylerjereddy/heatmap_diagonal
+
+In particular, commit hash `4712f54` from that project
+was used, and the precise timing and related details
+may be obtained from that project. In short, 32 bytes (1 per rank)
+were written at minor timing offsets in a Python mpi4py-based
+module on a single node of the LANL snow supercomputer
+(Intel skylake gold). Only, POSIX, DXT_POSIX, and
+(runtime) HEATMAP modules should be present. There is
+no *read* activity in the log (just 1 byte written
+per rank).
+
+The commit hash of the darshan main branch used to build
+the darshan runtime as `b4932a02a`.
+
+Here is the command line that was used on a 
+snow backend node:
+`mpirun -x DXT_ENABLE_IO_TRACE=1 -x DARSHAN_LOGPATH=/yellow/users/treddy/generated_darshan_logs -x DARSHAN_DISABLE_SHARED_REDUCTION=1 -x LD_PRELOAD=/yellow/users/treddy/darshan_install/lib/libdarshan.so -np 32 python heatmap_diagonal.py`
+
+Here are the steps there were followed to build
+the `darshan-runtime` on a snow backend:
+
+```
+module load gcc/9.3.0
+module load openmpi/3.1.6
+module load hdf5-parallel/1.10.7
+./prepare.sh
+cd darshan-runtime/
+./configure --prefix=/yellow/users/treddy/darshan_install --with-log-path-by-env=DARSHAN_LOGPATH --with-jobid-env=SLURM_JOBID --enable-hdf5-mod --with-hdf5=/usr/projects/hpcsoft/toss3/snow/hdf5/1.10.7_gcc-9.3.0_openmpi-3.1.6 CC=mpicc
+make
+make install
+```