Improve gmx sasa tests and clean up -or handling

Improve gmx sasa tests and clean up -or handling

Use the new xvg testing machinery to test the X axes of -oa and -or
(this was broken before a recent fix in release-5-0), as well as the
legends in all the xvg files.  Add one test that creates non-uniform
axes for the -oa and -or plots.

Simplify the code that produces the -or X axis by removing one state
variable.  Fix assertion in arraydata.cpp that was supposed to prevent
the original bug of incorrect axis initialization.

Make the reference data xslt files show the xvg data as HTML.
Improve handling of xvg values by preserving the string representation
from the xvg file in the reference data, which makes the HTML output
nicer and makes the xvg comparison work better when the precision of the
code changes.

Change-Id: I2cbe9646fddef705c8825c9c82de869d3205892c