Create a DummyGridder for use with Vector

[leouieda]

Description of the desired feature

The Vector class allows us to compose a multi-component gridder from scalar gridders (like gridding a 3D vector with 3 Spline instances). We can also use this to make a multi-component Trend and use it in a Chain. But sometimes we want to fit trends to some components and not others but still have a muilt-component Chain. Currently, there is no way of doing this. A way around would be to have a DummyGridder class that implements the gridder API but does nothing with the data. It's fit method should do nothing, predict should return an array of zeros, and filter should just return whatever inputs it got. Then we can do this:

components = (data.east, data.north, data.up)
# Only use a Trend for the East and North components
spline = vd.Chain([
    ("trend", vd.Vector([vd.Trend(degree=1), vd.Trend(degree=1), vd.DummyGridder()])),
    ("spline", vd.Vector([vd.Spline() for i in range(3)])),
])
spline.fit(coordinates, components)