Two improvements to API for BFAST feature

[oskooi]

Addresses two of the three recommendations in #2609 (comment) for improving the API of the recently added BFAST feature:

1. Eliminate the boolean parameter need_bfast.
2. Replace bfast_kbar with bfast_scaled_k which can be one of two types: Vector3 or None.

The third recommendation for setting the Courant factor automatically will be added in a separate PR (pending further tests).

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Codecov Report

Merging #2728 (ee9e68d) into master (ab18207) will decrease coverage by 0.01%. The diff coverage is 100.00%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #2728 +/- ## ========================================== - Coverage 74.07% 74.06% -0.01% ========================================== Files 18 18 Lines 5396 5395 -1 ========================================== - Hits 3997 3996 -1 Misses 1399 1399 ``` | [Files](https://app.codecov.io/gh/NanoComp/meep/pull/2728?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None) | Coverage Δ | | |---|---|---| | [python/simulation.py](https://app.codecov.io/gh/NanoComp/meep/pull/2728?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-cHl0aG9uL3NpbXVsYXRpb24ucHk=) | `77.37% <100.00%> (-0.01%)` | :arrow_down: |