Fix dimensionality bug in num_observations

[raphaelsaavedra]

length(model.system.y) works fine for the univariate case, but in multivariate cases where y is a matrix, length will return n_rows * n_cols which is not what we want. size(y, 1) works as intended for both vectors and matrices.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.30%. Comparing base (0b0198d) to head (1a76a29).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #343   +/-   ##
=======================================
  Coverage   91.30%   91.30%           
=======================================
  Files          36       36           
  Lines        3713     3713           
=======================================
  Hits         3390     3390           
  Misses        323      323           

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