Implement blowing slot boundary condition for arbitarary continuous x locations

[VanillaBrooks]

This implements a basic blowing boundary condition for a given start and stop location (grid points) in the config file. The blowing slot amplitude can only be configured through python currently.

Not all MPI nodes in python will have an array allocated (since its not possible to allocate a zero-sized array). MPI nodes in which the slot exists will have an array allocated at streams.mod_streams.blowing_bc_slot_velocity.

Configuration of the slot and input files is done from streams-utils configuration generator.

[VanillaBrooks]

with slot on [1,5]

velocity at idx 0 / local x 1 / global x 1 ::: 0.0
velocity at idx 1 / local x 2 / global x 2 ::: 0.75
velocity at idx 2 / local x 3 / global x 3 ::: 1.0
velocity at idx 3 / local x 4 / global x 4 ::: 0.75
velocity at idx 4 / local x 5 / global x 5 ::: 0.0

with slot on [148,152]

velocity at idx 0 / local x 1 / global x 151 ::: 0.75
velocity at idx 1 / local x 2 / global x 152 ::: 0.0
array shape for BC (2, 150)
velocity at idx 0 / local x 148 / global x 148 ::: 0.0
velocity at idx 1 / local x 149 / global x 149 ::: 0.75
velocity at idx 2 / local x 150 / global x 150 ::: 1.0