Add mem_size histogram

[olivialynn]

The second of three planned PRs for LSDB's #449.

Overall plan:

• First PR: change the directory names to mem_size_histogram and row_count_histogram
  • Rename histogram directory (histograms/ -> row_count_histograms/) #602
• Second PR: create the mem_size_histogram (but don't do anything with it)
  • 📍You are here
• Third PR: use the mem_size_histogram to do the partitioning

A little more detail:

1. Set up stage:
   1. We initialize the ResumePlan to have a concept of threshold_mode. The threshold mode defaults to row_count, but is set to mem_size if byte_pixel_threshold is in the input args (and is not None).
   2. We also specify some paths in ResumePlan: MEM_SIZE_HISTOGRAM_BINARY_FILE and MEM_SIZE_HISTOGRAMS_DIR.
   3. We run ResumePlan's gather_plan, which creates histogram directory(/ies) among other set up stuff.
2. Mapping stage:
   1. Here's where we map input files to Healpix pixels (via the call to map_reduce's map_to_pixels); and in doing so, we create the histogram. We pass threshold_mode to map_to_pixels, and if it's mem_size, we additionally make the mem_size histogram.
   2. We add memory size calculating method _get_mem_size_of_chunk and its two helpers _get_row_mem_size_data_frame and _get_row_mem_size_pa_table
3. Binning stage:
   1. No changes for now, except that we add an explicit parameter which_histogram to read_histogram to show that we're reading the row_count histogram. It's the default, but I wanted to include it for readability/safety.

[codecov]

Codecov Report

❌ Patch coverage is 95.65217% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.64%. Comparing base (fb2e1f3) to head (250fd09).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/hats_import/catalog/map_reduce.py	93.61%	3 Missing ⚠️
src/hats_import/catalog/resume_plan.py	97.29%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #619      +/-   ##
==========================================
+ Coverage   92.57%   92.64%   +0.06%     
==========================================
  Files          32       32              
  Lines        1926     1998      +72     
==========================================
+ Hits         1783     1851      +68     
- Misses        143      147       +4     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[delucchi-cmu]

Just two small comments. Otherwise LGTM!