Copying ModelArrays is too simplistic

[timspainNERSC]

Current the function ModelArray::operator=(const ModelArray&) delegates to the Eigen library to directly copy one Eigen data array (Eigen::Array or Eigen::Matrix) to another. If the two arrays do not have the same length in the second dimension, then the second dimension data is copied across the first dimension. That this is happening can easily be observed by adding a line

    if (m_data.size() != from.size())
        throw std::length_error(…);

to the function that copies the data. Running the model with this included causes a unhandled exception to be thrown during set up as a ModelArray containing DG components is copied into a HField Model array that does not expect them.