WFSS x1d, c1d preview images - use f_lambda

[stscijgbot-jwql]

Issue JWQL-273 was created on JIRA by Bryan Hilbert:

Conversations with WFSS folks indicate a preference for f_lambda units on the 1d spectral plots, rather than the Jy or MJy/sr units that the data are stored in.

[stscijgbot-jwql]

Comment by Bryan Hilbert on JIRA:

Jy -> F_lambda has been implemented. Still need to do MJy/sr -> F_lambda