Add MD17 dataset functionality

MD17.jl

@@ -0,0 +1,81 @@
+function __init__md17()
+    for name in ("benzene2017", "uracil", "naphthalene", "aspirin", 
+                 "salicylic", "malonaldehyde", "ethanol", "toluene")
+        DEPNAME = "MD17_$name"
+        URL = "http://quantum-machine.org/gdml/data/npz/md17_$name.npz"
+
+        register(DataDep(
+            DEPNAME,
+            """
+            Dataset: MD17 ($name)
+            Website: http://quantum-machine.org/gdml/#datasets
+            """,
+            URL
+        ))
+    end
+end
+
+"""
+    MD17(; name="aspirin", dir = nothing)
+
+The MD17 dataset is a collection of ab-initio molecular dynamics (MD) trajectories 
+for small organic molecules. It provides Cartesian coordinates (`R`), atomic numbers (`z`), 
+total energies (`E`), and atomic forces (`F`).
+
+The `features` are the Cartesian coordinates `R`. 
+The `targets` are the energies `E` and forces `F` as a `NamedTuple`. 
+The atomic numbers `z` are invariant across the trajectory and are stored in `metadata["z"]`.
+
+# Arguments
+
+- `name`: the name of the molecule. Available options are `"benzene2017"`, `"uracil"`, `"naphthalene"`, `"aspirin"`, `"salicylic"`, `"malonaldehyde"`, `"ethanol"`, and `"toluene"`. Default is `"aspirin"`.
+$ARGUMENTS_SUPERVISED_ARRAY
+
+# Fields
+
+- `metadata`: A dictionary containing the dataset name, path, and atomic numbers (`z`).
+- `features`: An array of Cartesian coordinates (`R`).
+- `targets`: A `NamedTuple` with `E` (energies) and `F` (forces).
+
+# Methods
+
+$METHODS_SUPERVISED_ARRAY
+
+# Examples
+
+```julia-repl
+julia> using MLDatasets
+
+julia> d = MD17(name="aspirin")
+dataset MD17:
+  metadata => Dict{String, Any} with 3 entries
+  features => 3×21×211762 Array{Float64, 3}
+  targets  => (E = 211762-element Vector{Float64}, F = 3×21×211762 Array{Float64, 3})
+```
+"""
+struct MD17 <: SupervisedDataset
+    metadata::Dict{String, Any}
+    features::Any
+    targets::Any
+end
+
+function MD17(; name::AbstractString="aspirin", dir = nothing)
+    valid_names = ("benzene2017", "uracil", "naphthalene", "aspirin", 
+                 "salicylic", "malonaldehyde", "ethanol", "toluene")
+    if name ∉ valid_names
+        throw(ArgumentError("Unknown molecule name: \$name. Available names are: \$valid_names"))
+    end
+
+    DEPNAME = "MD17_\$name"
+    data_dir = datadir(DEPNAME, dir)
+    filepath = joinpath(data_dir, "md17_\$name.npz")
+
+    data = read_npz(filepath)
+
+    features = data["R"]
+    targets = (E = vec(data["E"]), F = data["F"])
+
+    metadata = Dict{String, Any}("name" => name, "path" => filepath, "z" => data["z"])
+
+    return MD17(metadata, features, targets)
+end