Fix multivariate-normal parameter bug

[vallis]

Current code fails when specifying sigma as a matrix and size explicitly, due to a quirk in the numpy interface. Thanks to Aurelien Chalumeau for flagging this.

[codecov]

Codecov Report

Merging #317 (6eb2600) into master (dffb805) will decrease coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #317      +/-   ##
==========================================
- Coverage   87.97%   87.94%   -0.04%     
==========================================
  Files          13       13              
  Lines        2978     2978              
==========================================
- Hits         2620     2619       -1     
- Misses        358      359       +1     

Impacted Files	Coverage Δ
enterprise/signals/parameter.py	78.62% <100.00%> (ø)
enterprise/pulsar.py	92.21% <0.00%> (-0.26%)	⬇️

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