Support serving of general object attributes along with meta to the JLGui

[0Hughman0]

Currently only data that can get served to the JLGui... and therefore can be rendered in the children table are meta.

It would be extremely cool to allow more arbitrary attributes to appear in this table. This could also be a way of setting what appears by default in the table.

Potential implementation:

# project.py
class MyTier(BaseTier):

    __cassini_table_attrs__ = ['cookies']

    @property
    def cookies(self):
       return 'yummy'

Then in the server

data = _serialise_tier(data)
for attr in tier.__cassini_table_attrs__:
    data[attr] = getattr(tier, attr)

For example I found it useful to have a list of a samples datasets in the children_df.

This could be a more general way of allowing that:

class MySample(BaseTier):

    @cassini_table_attr
    @property
    def techniques(self):
       return [dset.name for dset in self]

This probably should live in the jupyer_cassini_server extension.

Perhaps we even move the jlgui.extend_project there and then have extend_project inject techniques as a starting point.