This commit adds new helper classes,
Opm::RestartIO::Helpers::CharArrayNullTerm<NChar>
Opm::RestartIO::Helpers::WindowedArray<T>
Opm::RestartIO::Helpers::WindowedMatrix<T>
the first of which represents a nul-terminated character string of
'NChar' user data (total array size is NChar + 1). Mainly intended
for representing Fortran-compatible characters arrays--especially
for ZWEL and similar restart vectors (e.g., ZGRP).
The second two simplify working with subsets of large linear arrays,
particularly those that represent N items per well, group, or
segment. The WindowedArray is a linear array of windows (N items
per window) packed into a std::vector<T> and the WindowedMatrix
additionally imposes a Row-by-Colum structure which is useful for
the case of N windows (each of size K) for each of M entities (e.g.,
K items for each of N connections for each of M wells).
