accept OpenMath objects without <OMOBJ>

[MarcoCostantini]

The OpenMath standard requires that an OpenMath object starts with the <OMOBJ> tag. However, some OpenMath implementations ( http://www2.macaulay2.com/Macaulay2/doc/Macaulay2-1.15/share/doc/Macaulay2/OpenMath/html/ , https://github.com/OpenMath/py-openmath , https://lurchmath.github.io/openmath-js/site/ ) produce objects without that tag. Hence it would be useful that Yacas accepts also OpenMath objects without the <OMOBJ> tag, in order to be able to interoperate with them.

In> FromString("<OMI>9</OMI> ")OMRead()
Out> OMError({"moreerrors","unexpected"},"String(1) : "encodingError:not an OMOBJ :XmlTag("OMI",{},"Open") 
"
")
In>