`get_projects` should accept a namespace parameter

[nleroy917]

It would be nice to be able to pass a namespace parameter to the get_projects method of PepAgent. This way I don't have to manually filter out namespaces that are not relevant when pulling them down and it saves on network latency.

Something like this:

projects = db.get_projects("geo")

[nleroy917]

Maybe this can be two things:

1. Just the project names, or
2. Full objects

Option 1:

>>> projects = db.get_projects("geo")
>>> projects
>>> ['GSE123456', 'GSE987654', ... ]

Option 2:

>>> projects = db.get_projects("geo", names_only=False)
>>> projects
>>> [ 
    'GSE12345': peppy.Project( ... ),
    'GSE98765': peppy.Project( ... )
]

[khoroshevskyi]

Added!