Setting photoz argument default to True

[tarraneh]

Automatically pulls photo-zs for PS1 and adds them to the PATH candidates table.

Note that this REQUIRES a MAST user account: https://mastweb.stsci.edu/mcasjobs/CreateAccount.aspx
And for MAST_CASJOBS_USER and MAST_CASJOBS_PWD environment variables to be set.

[SunilSimha]

In line 155's raise clause, I'd add an additional sentence for those who don't want to draw photoz's. e.g. "You need to ... get your credentials. Or set photoz=False in get_catalog."

[SunilSimha]

Approved