Add methods for building alternative units lists

based on:
- variable value
- attribute value
- other criteria


_Possible examples_
```cpp
openfluid::core::SpatialUnit* U

OPENFLUID_UNITS_VARIABLES_ORDERED_LOOP("TU",U,openfluid::core::AscendingLoop,"var.name")
{
  // ...
}

OPENFLUID_UNITS_ATTRIBUTES_ORDERED_LOOP("TU",U,openfluid::core::DescendingLoop,"attrname")
{
  // ...
}
``` 
