This commit adds a new container class,
PAvgDynamicSourceData
which wraps a single vector<double> which in turn backs source data
items for
- pressure values
- fluid mixture densities
- dynamic pore-volumes
at a collection of source locations-active cells or well reservoir
connections. We provide read-only and read-write accessors to the
underlying data items for a single source location through subscript
operator (read only) and named assignment (set(), read-write) member
functions. The latter is available only when the underlying range
of data values is mutable (e.g., SourceDataSpan<double>) as opposed
to immutable (e.g., SourceDataSpan<const double>).
