Use FITS-4 compliant date keywords in resulting maps

[wtbarnes]

The FITS-4 standard defines the following keys to specify the date(s) of the observations,

See Table 35 of this document.

In the resulting maps from fitting EIS spectra, the metadata have the keys DATE_BEG, DATE_END, DATE_AVG to describe the beginning, end, and average time of the observing interval, respectively. Note that these use a "_" character rather than the standard "-". In the EISMap source this is accounted for by overriding the relevant properties on the map itself. However, this seems like fixing a mistake that we can account for by just fixing the keys before producing the maps themselves by just replacing the relevant keys.