Checking features instead of access modes for non visual content

[mattgarrish]

In the epub techniques, the variable setup for non_textual_content_images in 4.2.2 is checking the wrong metadata property for the visual content indicators:

/package/metadata/meta[@property="schema:accessibilityFeature" and (text()="chartOnVisual" ...

It should be checking schema:accessMode not schema:accessibilityFeature. The value tests could also be shortened to avoid all the "or" connectors:

/package/metadata/meta[@property="schema:accessMode" and text() = ("chartOnVisual", "chemOnVisual", "diagramOnVisual", "mathOnVisual", "musicOnVisual", "textOnVisual")]

[mattgarrish]

Related to testing for one of a possible set of values, the textual_alternative_images variable could similarly be shortened to:

/opf:package/opf:metadata/opf:meta[@property="schema:accessibilityFeature" and text() = ("longDescription", "alternativeText", "describedMath")]

(Using text() for these, but see #562 for why it might be better to change them.)