BM1 and BM2 need some revisions

[wd15]

@tkphd, please expand on this

[tkphd]

For both BM1 (spinodal) and BM2 (Ostwald), part a imposes periodic boundary conditions but the initial condition is not periodic. This causes an initial transient as abrupt transitions at the boundaries rearrange themselves to align.

The solution is three-fold:

1. Make a note in the current variants to let people know this is by design.
2. Create new variants for both parts (a) with periodic initial conditions, without changing parts (b).
3. Introduce the initially-periodic variant as the benchmark of record, while preserving visibility of the original results.