magnum.fe update for FEniCS 1.5

We updated magnum.fe to work with FEniCS 1.5 which was released this week. Most notable changes of the new release are:

• The exchange-field computation is pulled in the ExchangeField class
• The newly introduced point-integral measure is now used for nodewise operations
• A Timer class is introduced for basic profiling

See the documentation for more details.