Allow other types into append

[arokem]

Right now, append only takes a TRX as input:

trx-python/trx/trx_file_memmap.py

Line 1239 in d2e5d9f

def append(self, trx: Type["TrxFile"], extra_buffer: int = 0) -> None:

For practical purposes, it would be useful to be allowed to append SFT and other types (ArraySequence, LazyTractogram) into an existing TRX.

[arokem]

For now, the pattern is:

0. Initialize an empty TRX (optionally preallocate, if you know how large this will be)
1. Create a LazyTractogram out of a batch of streamlines.
2. Create a small TRX out of that.
3. Append to the empty TRX you created in step 0
4. Delete the LazyTractogram.
5. Repeat steps 1-4 until done.
6. Optional: Resize if you preallocated (a lot more efficient if this thing is going to be big enough).

But this should eventually be superseded by resolving this issue.