tweak_initial_estimates: matrix handling

[barrettk]

Handling for matrix-type records. Specifically check for positive-definiteness and include handling for different formats of OMEGA and SIGMA matrices, such as:

• VARIANCE (default) orSTANDARD controls whether diagonals are taken as variance or standard deviation, respectively
• COVARIANCE (default) or CORRELATION controls whether the off-diagonals are taken as covariance or correlations, respectively
• CHOLESKY representations
• handling for SAME blocks and records defined like the one below:

  $OMEGA BLOCK(6)
  0.1
  0.01 0.1
  (0.01)x2 0.1
  (0.01)x3 0.1
  (0.01)x4 0.1
  (0.01)x5 0.1

  • Unclear if this will be supported vs error out at the moment.

closes https://github.com/metrumresearchgroup/bbr/issues/640