Make DataContainer name, unit and category attributes optional

[psomhorst]

When creating a data container, you have to supply a name, unit and category. Often, these are not necessary, but they make code quite verbose. Often, the name and category are (mostly) a copy-paste of the label. A lot of types of data don't require a unit.

Especially when creating a lot of data containers in a file, the three extra lines per data container add up.

I propose making the name default to a (capitalised?) copy of label, the unit default to None and the category default to "other".