Compat with Float32; reduction of preallocated; remove jld2 + ncdatasets deps

[JanJereczek]

I repeatedly bump into memory issues when running high resolution (nx = ny > 600). This is partly addressed here by reducing the number of arrays defined in PreAllocated and by deleting the 3D arrays from LayeredEarth.

[JanJereczek]

Additionally, I removed the deps to JLD2 and to NCDatasets. The former is motivated by the fact that isostasy_data should not be julia-specific. The latter is motivated by the fact that we already need NetCDF.jl and hence we can easily remove NCDatasets.jl, which has the additional perc of being more legible.

[JanJereczek]

The code now includes a test with everything being Float32. This works and will avoid future issues about memory. In particular, this allows to run high resolution simulations on GPU!

[JanJereczek]

This addresses #119, #103, #120!