Move scipy to optional requirements

[rly]

Motivation

scipy is used only by the CSRMatrix data type, which is currently rarely used. To reduce the chance of dependency conflicts and reduce the install time, I propose we make scipy an optional requirement.

This is a breaking change since users using the CSRMatrix type will need to install scipy separately.

Merging into new hdmf_4.0 branch.

Checklist

• Did you update CHANGELOG.md with your changes?
• Does the PR clearly describe the problem and the solution?
• Have you reviewed our Contributing Guide?
• Does the PR use "Fix #XXX" notation to tell GitHub to close the relevant issue numbered XXX when the PR is merged?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.79%. Comparing base (539ecf4) to head (7942faa).
Report is 1 commits behind head on hdmf_4.0.

Additional details and impacted files

@@             Coverage Diff              @@
##           hdmf_4.0    #1140      +/-   ##
============================================
+ Coverage     88.78%   88.79%   +0.01%     
============================================
  Files            45       45              
  Lines          9778     9790      +12     
  Branches       2777     2779       +2     
============================================
+ Hits           8681     8693      +12     
  Misses          780      780              
  Partials        317      317              

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[mavaylon1]

Code Coverage