Add empty constructors plus initialise() methods with all parameters for all Discrete estimators

[jlizier]

This will allow dynamic reuse of these calculators for different bases. Seems to be useful for calls from python, since jpype isn't properly garbage collecting calculators that are no longer referenced from Python (so their memory persists) -- if we can reuse them with different bases, their memory should be cleaned up ok.

This issue will provide an important enabler for #66 as well