read in existing auxiliary files into msh obj, updates to make_bc

[WPringle]

Some additions for sponge zone construction, reading auxiliary files, writing offset63, swanoutput namelist, update Make_f5354 help and min_depth option.

[WPringle]

Sponge example:

% input files
mesh_file = 'my_mesh.mat'
tpxoh = 'h_**_tpxo9_atlas_30_v5.nc';
tpxou = 'u_**_tpxo9_atlas_30_v5.nc';

% load the mesh
load(mesh_file)

% add the sponge layer (width 10% of M2) 
sponge_opt = 1; % for tidal boundary conditions
m = Calc_Sponge(m,[12.42*3600, 0.1],sponge_opt);

%% Make the fort.15 struct
TS = start_time; %[date-string]
TE = end_time; %[date-string]
DT = time_step; %[s]
CONST = "major8"; % tidal constituent information
m = Make_f15(m,TS,TE,DT,'const',CONST,'tidal_database',tpxoh);

% add the sponge info for this tidal
m = Make_f5354(m, tpxoh, tpxou);
% write out mesh files..
write(m,'sponge_mesh')

[krober10nd]

I like the ability to read in aux files using the msh.read() perhaps this could be added to the README as a code snippet and a short couple sentences?

[krober10nd]

Please preserve the prior recently added items on the checklist.

[krober10nd]

The only other comment pending is regarding preserving the previous entries in the changelog. Otherwise good to go.