`table_diff` should allow more control on how diffs are computed

[chipkent]

Currently table_diff is extremely rigid in how diffs are computed. There are strict predetermined tolerances, the tolerances apply to all columns, etc. To make table_diff more useful:

• The way floating point diffs are computed needs to be changed. Julia's isapprox function is what we want to emulate. It allows both absolute and relative tolerances to be checked at once. It also allows NaN behavior to be set. https://docs.julialang.org/en/v1/base/math/#Mathematical-Functions. https://www.jlhub.com/julia/manual/en/function/isapprox
• For limits:
  1. We need absolute and relative limits.
  2. We need a dict to supply by-column limits.
  3. We need "default" limits that apply if no by-column limits are specified.