Allow missing tiles in ConcatDiskArray

[felixcremer]

When we mosaic DiskArrays together on a >2D grid they're chunks have to align so that the resulting ConcatDiskArray can be constructed.
Sometimes there are missing tiles and these have to be filled with some value or missing.
For this we introduced ChunkedFillArray in DiskArrayTools so that the chunks can be aligned. But it would be better to allow a missing value or a fill value in the Array of DiskArrays that is the input to ConcatDiskArray.

ConcatDiskArray([1, DiskArray(rand(10,10); DiskArray(rand(10,10), 2] ) should work and would fill the upper box with 1 and the lower box with 2.

[rafaqz]

Makes sense, just needs a fill keyword like PaddedDiskArray and handling for readblock where there are missing arrays. I guess that's also kind of similar to PaddedDiskArray