Rename `qbx_forced_limit` to be method-independent (i.e. not QBX-specific)

[inducer]

• It's legitimate to ask, "which limit would you like".
• Not every method can compute all limits (most just return the principal value/finite part/"average")
• For QBX in IE solves, it's convenient to "compute both limits, average, then apply the jump relation by hand". That's not necessarily true for other methods either.

cc @isuruf