Update default config/allow config

[mikeAdamss]

Currently databaker defaults the value and marker columns to OBS and DATAMARKER respectively (which the DEs then have to change).

This is a callback to the old WDA days and is defined here:

databaker/databaker/structure_csv_default.py

Lines 8 to 9 in 13929bb

	('observation', "OBS"),
	('data_marking', "DATAMARKER"),

Ideally, we should refactor this to take a proper config file (eg use Value and Marker) for new pipelines without changing it globally (otherwise all the old pipelines will break).

For mvp, it does support including a structure_csv_user.py in your databaker install which will be used in preference to structure_csv_default.py (so config, just config via py file).

Ideally we'd want a solution where this can be configured via an environment variable (so different pipelines have different config) to keep everything backwards compatible.