This PR updates and simplifies the implementation of implicit solvers of the form:

rho [(u(n+1) - u(n))/dt + A(n+1/2)] = - G p(n+1/2) + mu L (u(n+1) + u(n))/2 + S F[X(n+1)]
D u(n+1) = 0
(X(n+1)-X(n))/dt = J u(n+1)

with

A(n+1/2) = time step-centered approximation to the convective term
S = IB spreading operator
J = IB interpolation operator
F[X] = IB force evaluated using IB positions X

as well as the "direct forcing" form:

rho [(u(n+1) - u(n))/dt + A(n+1/2)] = - G p(n+1/2) + mu L (u(n+1) + u(n))/2 + S F(n+1)
D u(n+1) = 0
F(n+1) = -eta J u(n+1)

IBAMR Pull Request Checklist

• Does the test suite pass?
• Was clang-format run?
• Were relevant issues cited? Please remember to add Fixes #12345 to close
  the issue automatically if we are fixing the problem.
• Is this a change others will want to know about? If so, then has a
  changelog entry been added?
• If new data structures have been added to a class then have they been set
  up appropriately for restarts? If so, ensure that the restart version
  number is incremented.
• Does this change include a bug fix or new functionality? If so, a new test
  or tests should be added. New tests should run quickly (less than a minute
  in release mode). If possible, an older test should gain a new option so
  that we do not need to compile more test executables.
• Did you (if your account has permission to do so) set relevant labels on
  GitHub for the pull request?