Incorrect x_dot (velocity) field with some steppers

[ikalash]

This is a repeat of an Albany-LCM issue (sandialabs/LCM#96) because it is present also in Albany.

For some Tempus steppers, the x_dot field is all 0, when it should be non-zero.

Issue 1.) can be reproduced by running https://github.com/sandialabs/LCM/blob/main/tests/TransientHeat2D/input_be.yaml and https://github.com/sandialabs/LCM/blob/main/tests/TransientHeat2D/input_rk.yaml for the TransientHeat2D problem, where the DBCs applied are all 0 except at the bottom of the domain, where a 1*t time-dependent DBC is prescribed. The x_dot should therefore be 1.0 for all times at the bottom boundary. The correct behavior is observed when using a Backward Euler stepper, but not when using an RK Implicit Midpoint stepper, as shown below.

I have talked to @ccober6 about this and the issue is a "feature", namely that Tempus doesn't have xdot implemented for all steppers. I don't think xdot is really needed in Albany / for ALI, so I said this is not urgent to fix, but may change once we start doing dynamics in Albany.

Another issue that may impact Albany and is related is that xdot values are solved for at all the dofs, not just the unconstrained ones, as should be done. This can result in an incorrect value of xdot at the boundary. For more detail, please see sandialabs/LCM#96 and the discussion therein.

Opening issue here so we document and don't forget about the problem.