Plot wideband DMs and DM residuals in pintk

[abhisrkckl]

Implements #1430

Three new plotting options: WB DM, WB DM res, WB DM err. These are disabled for narrowband data.

Also fixes a bug introduced in #1663, and fixes a broken import.

[codecov]

Codecov Report

Attention: 33 lines in your changes are missing coverage. Please review.

Comparison is base (7c14b7b) 68.47% compared to head (b5d095d) 68.40%.
Report is 1 commits behind head on master.

Files	Patch %	Lines
src/pint/pintk/plk.py	5.88%	32 Missing ⚠️
src/pint/pintk/pulsar.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1534      +/-   ##
==========================================
- Coverage   68.47%   68.40%   -0.08%     
==========================================
  Files         104      104              
  Lines       24305    24336      +31     
  Branches     4337     4345       +8     
==========================================
+ Hits        16644    16646       +2     
- Misses       6574     6603      +29     
  Partials     1087     1087              

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

[abhisrkckl]

DM Measurements:

DM Residuals:

DM Errors:

These new radio buttons are disabled for narrowband data.