Add compare command

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Procedure: Compare the curves from two ULTRA files. This function expects the names of two files which should have comparable curves. The function diff-measure is mapped over each pair of curves (one from each file) with the following result. For the nth curve from each file a fractional difference measure is computed and if the average value of the fractional difference exceeds the error tolerance, the pair of curves is displayed along with the fractional difference and its integral along with a prompt consisting of the curve number, the label of the first curve and the value of the average fractional difference. The user can respond with “y” to indicate acceptance of the result or “n” to indicate non-acceptance of the result. At the completion of the process, a log file ending with “.chk” is written summarizing the curves which differed and were not accepted. This can be used to automate the comparison of simulation code results from one run to another. The optional error tolerance defaults to 10-8. Note: compare has the side-effect of erasing all curves from the screen and killing any curves on the menu.
Usage: compare file-name1 file-name2 [tolerance]

No files or tests are named. Start by locating the command-dispatch entry point and existing ULTRA-file, curve-comparison, and display commands, then trace how prompts and log files are handled. Done means compare accepts two files and an optional tolerance, reports unaccepted differing curves, and writes the specified .chk summary.

Written by the indexing model from the issue text.