Use SwitchModulus for ScaleAndRoundPOverQ

[ZenithalHourlyRate]

Fixes #992

With changes to bfv-mult-bug.cpp using EXTENDED encryption mode, we can observe the following trace before and after this commit

Before this commit

$ ./bin/extras/pke/bfv-mult-bug
Plaintext #1: (1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, ... )
Plaintext #2: (3, 2, 1, 4, 5, 6, 7, 8, 9, 10, 11, 12, ... )

Results of homomorphic computations
#1:      (1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, ... )
#1 * #2: (3, 4, 3, 16, 25, 36, 49, 64, 81, 100, 121, 144, ... )
logQ: 2220
noise: 69.265
logQ: 2220
noise: 5.70044

After

$ ./bin/extras/pke/bfv-mult-bug
Plaintext #1: (1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, ... )
Plaintext #2: (3, 2, 1, 4, 5, 6, 7, 8, 9, 10, 11, 12, ... )

Results of homomorphic computations
#1:      (1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, ... )
#1 * #2: (3, 4, 3, 16, 25, 36, 49, 64, 81, 100, 121, 144, ... )
logQ: 2220
noise: 69.4396
logQ: 2220
noise: 4.58496

Note that the noise in the last line is one bit smaller, as expected.