Package changes from previous ecolRxC version 0.1.1-10

Compared to version 0.1.1-10 of ecolRxC, the new version:

• fixes a bug in the estimation of confidence transition probabilities when method = "Thomsen". As a result, more accurate estimates are obtained on average.
• fixes a bug in the estimation of confidence intervals for the transfer probabilities when method = "IPF".
• includes a new argument (regions) to split the set of units into subsets, applying the selected ecolRxC specification separately to each subset.
  
• includes a new argument (ref.combination) to specify how unit table estimates should be combined to generate the default output when method = "Thomsen" and reference = NULL.
• includes, as new outputs, the bounds implied by the observed data for the transfer probabilities.
• offers a new method to estimate confidence intervals for the transfer probabilities adapted to this setting, based on Pavía-Miralles (2005) https://www.jstor.org/stable/27590658.
• computes bivariate Gaussian probabilities using mvtnorm::pmvnorm() to acelerate the process.
• translates some internal functions to C++ to accelerate computations.