Bug: snapshot fails when instantiating InstrumentBase

[astafan8]

The problem occurs when one wants to subclass from InstrumentBase (or instantiate it). The reason to do so is to create a "virtual" instrument, and to avoid the overhead of Instrument class related to the register of instances.

The problem is that the snapshot functionality of InstrumentBase raises an exception saying "the object does not have attribute _meta_attrs". This is because snapshot_base uses it, but for some reason _meta_attrs is only defined in the Instrument class. This is not correct, hence this PR.

Changes:

• declare _meta_attrs in InstrumentBase instead of Instrument
• add tests for snapshotting InstrumentBase
• modify Instrument test to ensure snapshotting still works for it

[codecov]

Codecov Report

Merging #1209 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1209      +/-   ##
==========================================
- Coverage   80.16%   80.16%   -0.01%     
==========================================
  Files          49       49              
  Lines        6762     6761       -1     
==========================================
- Hits         5421     5420       -1     
  Misses       1341     1341

[WilliamHPNielsen]

It's not so clear to me what is gained by instantiating InstrumentBase directly, but irrespective of that, this refactor seems sensible to me.

[WilliamHPNielsen]

A-OK from me.

[astafan8]

yeah, it is more for subclassing InstrumentBase.