PaillierContext in CLI

[mpnd]

Currently CLI uses one type of Paillier Context, a signed context with 32 bits precision and default base.
Ideally, in addition to a default Context, users should be able to define the Paillier Context set up (regarding the precision, signed/unsigned and base).

Another important note, the current precision used in default Context severely limits the range of number that can be encrypted/decrypted given the default keysize.

[hardbyte]

How about we just add as new cli options:

--unsigned
--base [base]
--precision [precision]