Add Option to Metis Data Source to obtain profiles on demand.

[Mateasek]

All available profiles are now loaded when the metis objects are initialized. This is optimal when the data are being loaded from a single file. Because the data can be read from various sources it would make sense to give user the possibility to chose whether the profiles are loaded during the initialization of on demand.

This could be handled by the data source class, together with raising exceptions when the requested profile is not available.