Feature/ot singlet compatibility [MEDIUM]

[daknuett]

• ot_singlet was vastly incompatible with other ots.
• For future work on ot embeddings all ots must be treated consistently.

This pullrequest changes the following:

• ot_singlet is instantiated like all ots now.
• ot_singlet and subclasses are singletons now.
• value.otype == g.ot_singlet and value.otype.data_otype() == g.ot_singlet continue working as before.

[daknuett]

The new changes provide lattice.otype.is_singlet which is True for ot_singlet and all derived classes, and False for all other classes, including ot_base.

[lehner]

Closing this since it should be contained in discussed improvements of type system.