[entityseed] add default constructor to interface class

see FS#1221
[entityseed] add isValid to the facades class

see meeting minutes of the dev meeting 2013
[test][entityseed] update test to latest interface changes

the EntitySeed is now default constructable.
The validity of a seed can be checked with isValid()
[sgrid] add EntitySeed default constructor and method isValid
[entityseed] make impl() a protected member

the impl() method of the EntitySeed is now protected.
It can be accessed via the Grid. Thus the Grid is now a friend.
[entityseed] when using experimental grid extensions we grant direct access
[entityseed] add mutable access to impl()

see FS#1346
[yaspgrid] add EntitySeed default constructor and method isValid
[onedgrid] add EntitySeed default constructor and method isValid
[albertagrid] add EntitySeed default constructor and method isValid
[uggrid] add EntitySeed default constructor and method isValid
[alugrid] add EntitySeed default constructor and method isValid
[geometrygrid] add EntitySeed default constructor and method isValid
[bugfix][test] fix assertion for entityseed test

we checked the wrong seed for validity
[bugfix][sgrid] fix EntitySeed::isValid

we had it the wrong way around
[bigfix][albertagrid] fix isValid() implementation
Merge branch 'master' into feature/FS1366-FS1346-FS1221_entityseed_default_constructor_and_impl_private
Merge remote-tracking branch 'origin/master' into feature/FS1366-FS1346-FS1221_entityseed_default_constructor_and_impl_private