Separate Core API from Features on DataCard

[rtxm]

All changes have consequences in other parts of the code that must be updated.

Properties

Core properties:

• title
• index
• author?
• creation_date
• uid as uuid to replace adhoc 'card_XX' id?

Property to remove:

* column; only column shoud have a handle on cards. needed for some joins

All other properties are considered belonging to features (including members). Group them and comment accordingly.

Methods

create_card and delete_card belong to DataColumn, as instance methods.

Rename remove_board_member to remove_member.

All other methods are considered belonging to features. Group them and comment accordingly.

[rtxm]

#138, #126