Fix for irregular grid in FindLinearIdx

[WPringle]

Testing for irregular DEM grid in FindLinearIdx function in order tointerpolate properly using cell-averaged method for irregular DEM grid

[krober10nd]

So it looks like there's three general cases:

1. Uniform grid DEM with both axes featuring same grid spacings.
2. Varying grid spacings in x and y axes.
3. Varying grid spacings along the x/y axis itself potentially at the same time (this is the irregular case that did not work).

I'll test this this weekend and get back to you.

[WPringle]

Yes, that's right.

I made a test (just modification of Example2) using the DEM used in the issue #203 and it seemed to fix that alignment using the updated varying grid spacings along the x/y axes.

[krober10nd]

Could you send me this test please?

[krober10nd]

Thinking about this logic, seems maybe a bit faulty.

if max(dx) ~= min(dx)

This also could be the case for dy.

I would make this

if max(dx) ~= min(dx) || max(dy) ~= min(dy)

[WPringle]

True that's a good change

[krober10nd]

what I would like to see is a test in the tests folder that demonstrates all three cases that I enumerated below work.
For example, generate a DEM from an analytical function and then build the mesh over and over again with finer solution and compare the interpolated bathymetry with the analytical function.

[krober10nd]

Given that this problem has now appeared three times, it's important this doesn't happen again.

[WPringle]

fair enough, I can work on that.

[krober10nd]

Let me know if you need any help

[WPringle]

Made a test using a subset of the CUDEM with different resolution types. Please check it out

[WPringle]

Made sure test does not pass with the previous interpolation method.