Add USet class

[bleudev]

Description of new feature

I suggest to add USet and UInterval classes for simplyfing working with sets and intervals.

from ufpy import USet, inf

# Sets
USet.U = {1, 2, 3, 4, 5, 6, 7, 8, 9, 12, 74} # set universal (in sys.environ)

a = USet(1, 9, 2)
b = Uset(9, 1, 12, 74)

a & b # u{1, 9}
a | b # u{1, 2, 9, 12, 74}
a - b # u{2}

-a # not op; eq. of "USet.U - a"; u{3, 4, 5, 6, 7, 8, 12, 74}
# Note: if in b there are elements, that not in USet.U, USet.U += {this elements} automaticly

# From bool logic
a > b # a -> b; implication

# Intervals
a = UInterval(-inf, 5) # u(-∞, 5)
b = UInterval([3], +inf) # u[3, +∞)

a | b # u(-∞; +∞)
a & b # u[3; 5)
a - b # (-∞; 3)

-a # u[5, +∞)

Use Case (optional)

In Math exercises.

Suggest solution (optional)