Configuration to map dimension, measure, or attribute values to use a prefered namespace

[csarven]

It should be possible to configure dimension, attribute, or measure values based on the concept that's defined in the config file to any namespace. For example, ECB uses the TIME_FORMAT concept for an attribute (which is a isTimeFormat) where the values in the dataset for this attribute is a member of CL_TIME_FORMAT in http://sdmx.org/wp-content/uploads/2009/01/02_sdmx_cog_annex_2_cl_2009.pdf. So, in this case, use the appropriate concept from sdmx-code:timeFormat.

Relates to issue #34 but this is for values instead of properties.