Csv file fixes

[talister]

Change Description

• Allows the ability to pass units to read_element for use when reading CSV files so nm are not assumed.
• Adds ZnSe transmission data
• My PR includes a link to the issue that I am addressing

Solution Description

Checks for a passed value of wave_unit= that is not u.nm and passes the new unit to specio.read_ascii_spec() if this is the case

Code Quality

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Bug Fix Checklist

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• My change includes a breaking change
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