Refactor and enhancement of physics-side nudging functionality

Goal: Within the existing (MCT) infrastructure, revise the
physics-side nudging module to remove existing I/O
bottlenecks and to implement run-time interpolation
of data values directly from reanalysis products onto
a given model grid decomposition.

(1) The physics-side nudging module does serial read of data
and then distributes the data to the processing elements.
Revise this module to replace these routies with infld()
calls that will read the data via PIO.

(2) For backward compatability, the current infld() routine
can read datasets containing values on the same model grid.
Create an enhanced version of infld() which can read in
2D/3D data from a rectilinear (reanalysis) grid and
interpolate values horizontally/vertically onto the given
PE's grid points.

(3) Revise the nudging module so that it is configured to read
in data from currently 'known' reanalysis datasets.
{first ERAI, then ERA5}

The revisions must provide a means by which users can
add additional datasets or reanalysis products.

(Limitation: only netCDF files for now.)