SSO improvements

[JoanneBogart]

The initial SSO code has some rough edges.

• Use of streaks. It probably is the case that we'll always want to render SSOs as streaks, not point sources. When that's confirmed, eliminate streak option (now defaulted to True) in SsoObject.get_gsobject_components
• Don't do division by cos(dec) in SsoObject.get_gsobject_components. Either (preferred) request that Sorcha output not include this factor in ra rate or do the division just once when creating the main sky catalog
• SED file (currently an sqlite3 db) should be in standard text format, readable by galsim. It could be included in the skyCatalogs package. (Will it always be the case that the same SED is used for all SSOs?)
• Move hard-coded default path for SSO input (from Sorcha run) to the script create_sc.py

[JoanneBogart]

Although streak is the default, there are situations where it's impossible to render a particular SSO as a streak, in which case we fall back to point source.

[JoanneBogart]

Last two items are addressed in PR #120