This commit introduces a new helper class, LinearisedOutputTable,
that is oriented towards managing tabular data such as the PVT or
saturation functions in an ECL result set (typically, .INIT file).
The class knows about row padding and the column oriented nature of
Fortran-like tables.
While here, and in anticipation of adding output of tabulated
saturation functions, also provide a mechanism for calculating
slopes of piecewise linear interpolants. The ECL format stipulates
that function derivatives be output along with the actual table
data for most functions.
