Non-uniform boundary conditions

[marinlauber]

This pull request adds the capability of using space and time-varying inflow boundary conditions.

The major change is that now the U::Function passed to the Flow is of the form U(i,x,t) where i is the component, x the location in physical space and t the time. The flow is then accelerated correctly and the boundary conditions on the velocity field are applied correctly.

As of commit 2766de4 all the examples run (when they are adjusted properly). Once this one is merged, I will update the examples.

Results of the test with 2766de4

[ Info: Test backends: Array, CuArray
Test Summary: | Pass  Total   Time
util.jl       |   42     42  18.7s
Test Summary: | Pass  Total   Time
Poisson.jl    |   14     14  17.8s
Test Summary:        | Pass  Total  Time
MultiLevelPoisson.jl |   13     13  7.3s
Test Summary: | Pass  Total   Time
Flow.jl       |   22     22  12.2s
Test Summary: | Pass  Total  Time
Body.jl       |    3      3  0.1s
Test Summary: | Pass  Total  Time
AutoBody.jl   |   16     16  5.7s
Test Summary:        | Pass  Total  Time
Flow.jl periodic TGV |    2      2  4.9s
Test Summary: | Pass  Total   Time
ForwardDiff   |    2      2  26.8s
Test Summary:                      | Pass  Total  Time
Flow.jl with increasing body force |    2      2  8.6s
Test Summary: | Pass  Total   Time
Metrics.jl    |   37     37  13.1s
Test Summary: | Pass  Total  Time
WaterLily.jl  |   30     30  6.4s
Test Summary: | Pass  Total   Time
VTKExt.jl     |   28     28  14.7s
     Testing WaterLily tests passed 

Results of the allocation test fail for now.