Use model singletons

[msmolens]

Modernize code to use Girder model singletons [1].

For example, replace

self.model('folder').load(...)

with

from girder.models.folder import Folder
Folder().load(...)

and replace

self.model('phase', 'covalic').load(...)

with

from .models.phase import Phase
Phase().load(...)

[1] https://girder.readthedocs.io/en/latest/api-docs.html#models

[brianhelba]

Fixed in #249