guard memory use in VolumeMapping.get_mapping()
guard memory use in VolumeMapping.get_mapping()

- the problem cannot be solved if cmap.bfg takes the whole memory
- IGMapping not treated due to limited use (single patch support only)

sfepy/sfepy,sfepy/sfepy,rc/sfepy,rc/sfepy,BubuLK/sfepy,BubuLK/sfepy,vlukes/sfepy,rc/sfepy,BubuLK/sfepy,vlukes/sfepy,sfepy/sfepy,vlukes/sfepy