Refactored MineField into 3 classes: Abstract base class Field and
subclasses NumberField and MineField. Instances of MineField are always
armed and have no adjacent Mines. They can only be retrieved through
factory methods. NumberFields are never armed and can have zero or more
adjacent Mines. Both classes are fully immutable and will thus return
new Objects on being operated on. Furthermore armed Fields (that is
instances of MineField) now switch into the state "triggered" rather
than being uncovered when the uncover operation is executed on them.

MineFieldTest tests the MineField subclass of Field with the above
mentioned changes. A test for NumberField still needs to be written.