Increase chunk size based on memory

[JoeZiminski]

When writing to binary, a small chunk size is currently used. In general I think the less chunks the better, to avoid edge effects. A such it might be a good idea to a large possible chunk size that memory will allow (e.g. say use 70% of available memory)

[JoeZiminski]

#108

[JoeZiminski]

Scaling the memory was not as simple as possible. First, SI's memory tracking is tagged to undergo so improvement and is not trivial to implement, so getting a good estimate of memory used during pre-processing is not easy.

Nonetheless, a rough guess could be used, taking the maximum itemsize (for float 64 used in some preprocessing steps) and a 2-3 times multiplier based on dev feedback.

See #108 for a first implementation. The current blocker on this was finding the available memory across different settings.

1. psutil.virtual_memory().available did not give accurate memory on SLURM nodes, giving a much higher value than requested (e.g. request 40GB it is showing 380 GB
2. slurmio always returned 16GB even if 40 GB was requested.
   e.g.

(spikewrap) jziminski@gpu-380-14:/ceph/neuroinformatics/neuroinformatics/scratch/jziminski/ephys/code/spikewrap$ sacct  --format="MaxRSS, MaxRSSNode"
    MaxRSS MaxRSSNode
---------- ----------
 45941200K gpu-380-14
     7992K gpu-380-14
    17116K enc3-node4

but

>>> SlurmJobParameters().requested_memory
16
>>> SlurmJobParameters().allocated_memory
16000000

Once this is resolved it would be possible to expose a argument 'fixed_batch_size' that allows the user to fix a batch size. Otherwise, use 70% or so of available memory.