Function for streamlining the creation of a Spectral_data instance

[jdavidpeery]

Is your feature request related to a problem? Please describe. Currently, you have to write out a lot in order to create a new Spectral_data object. This can be somewhat redundant and cumbersome when slightly altering a previous Spectral_data object, such as in #888

Describe the solution you'd like A new function that sets some metadata terms to default values, and/or adding default values to the Spectral_object class