ENH: robuster handling inv() of singular tensor for finite-area LSQ (#2724)

- with the current handling of small edges (finite-area), the LSQ
  vectors can result in singular/2D tensors. However, the regular
  2D handling in field inv() only detects based on the first element.

  Provide a 'failsafe' inv() method for symmTensor and tensor that
  follows a similar logic for avoiding zero determinates, but it is
  applied on a per element basis, instead of deciding based on the
  first field element.

  The symmTensor::inv(bool) and tensor::inv(bool) methods have a
  fairly modest additional overhead.

- unroll the field inv() function to avoid creating an intermediate
  field.  Reduce the number of operations when adjusting/re-adjusting
  the diagonal.