Patch refactor

[d-chambers]

Description

This is a HUGE pull request. It refactors many (most) of DASCore's internals. Despite the immensity of the changes, I don't think it introduces any significant backward incompatibilities. Here are a few highlights:

1. Implements numpy-based coordinates (dascore.core.coords/coordmanager) rather than using xarray.DataArray. This gives us more control over the patch/data/coordinates and simplifies and speeds up lots of other code. Xarray is now an optional dependency.
2. Implements first-class support for units using the pint library.
3. Refactors merging/chunking for fairly large performance improvements.
4. Refactors most of the parsers to use DASCore coordinates and ensure their read/scan operations return the same min/max values.
5. Better string representation of objects

There are also significant speed ups as shown in the following snapshot from the asv benchmarks. Patch, merge, chunk, and select are now significantly faster. Generating string representations for Patches and Spools, however, is not about 60% slower. This is largely due to the addition of rich styling, and probably wont be a burden for most codes since calling __str__ on a large number of DASCore objects is not likely a common use.

Here are a few things left to do:

• Full doc page on coordmanager (probably in dascore.core.coordmanager)
• More tests with units and processing functions; particularly pass_filter and taper.
• Merge tests for patches with various types of coordinates (monotonic, evenly sampled, no restraints)
• Update docs for Patch to explain string repr. and coordinates
• Check coverage, clean up dead code.
• Add a minimal deps test to CI, probably just for linux.
• Create a release before merging, so there is a nice checkpoint to refer users to if anything breaks (while we fix it)

Checklist

I have (if applicable):

• referenced the GitHub issue this PR closes.
• documented the new feature with docstrings or appropriate doc page.
• included a test. See testing guidelines.
• your name has been added to the contributors page (docs/contributors.md).
• added the "ready_for_review" tag once the PR is ready to be reviewed.

[codecov]

Codecov Report

Merging #180 (611519e) into master (26d7961) will increase coverage by 1.00%.
The diff coverage is 99.44%.

@@            Coverage Diff             @@
##           master     #180      +/-   ##
==========================================
+ Coverage   96.77%   97.78%   +1.00%     
==========================================
  Files          58       64       +6     
  Lines        3627     4916    +1289     
==========================================
+ Hits         3510     4807    +1297     
+ Misses        117      109       -8     

Flag	Coverage Δ
unittests	97.78% <99.44%> (+1.00%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
dascore/viz/__init__.py	100.00% <ø> (ø)
dascore/viz/wiggle.py	79.59% <ø> (ø)
dascore/io/quantx/core.py	96.55% <50.00%> (-3.45%)	⬇️
dascore/utils/patch.py	97.04% <95.12%> (-0.79%)	⬇️
dascore/core/spool.py	98.96% <95.74%> (+0.21%)	⬆️
dascore/proc/units.py	97.61% <97.61%> (ø)
dascore/utils/models.py	98.13% <98.13%> (ø)
dascore/utils/display.py	98.91% <98.91%> (ø)
dascore/__init__.py	100.00% <100.00%> (ø)
dascore/clients/dirspool.py	100.00% <100.00%> (ø)
... and 32 more

... and 2 files with indirect coverage changes

[d-chambers]

All green and checklist complete.