feat(io): add Abaqus model download utility

New 43-line download utility:
- abaqus_download(): download Abaqus models from internet
- Uses environment variables: ABAQUS_DOWNLOAD_URL, ABAQUS_DOWNLOAD_DIR
- Checks if file already exists before downloading
- Supports dry-run mode for testing
- Required by AbaqusReader module for example model downloads

Provides utility for downloading Abaqus example models from documentation.

src/io/abaqus_download.jl

@@ -0,0 +1,43 @@
+# This file is a part of JuliaFEM.
+# License is MIT: see https://github.com/JuliaFEM/AbaqusReader.jl/blob/master/LICENSE
+
+"""
+    abaqus_download(model_name; dryrun=false)
+
+Download ABAQUS model from Internet. `model_name` is the name of the input
+file.
+
+Given some model name from documentation, e.g., `et22sfse`, download that
+file to local file system. This function uses environment variables to
+determine the download url and place of storage.
+
+In order to use this function, one must set environment variable
+`ABAQUS_DOWNLOAD_URL`, which determines a location where to download. For
+example, if the path to model is `https://domain.com/v6.14/books/eif/et22sfse.inp`,
+`ABAQUS_DOWNLOAD_URL` will be the basename of that path, i.e.,
+`https://domain.com/v6.14/books/eif`.
+
+By default, the model will be downloaded to current directory. If that is not
+desired, one can set another environment variable `ABAQUS_DOWNLOAD_DIR`, and
+in that case the file will be downloaded to that directory.
+
+Function call will return full path to downloaded file or nothing, if download
+is failing because of missing environment variable `ABAQUS_DOWNLOAD_DIR`.
+"""
+function abaqus_download(model_name, env=ENV; dryrun=false)
+    path = get(env, "ABAQUS_DOWNLOAD_DIR", "")
+    fn = joinpath(path, model_name)
+    if isfile(fn)  # already downloaded
+        return fn
+    end
+    if !haskey(env, "ABAQUS_DOWNLOAD_URL")
+        error("ABAQUS input file $fn not found and `ABAQUS_DOWNLOAD_URL` not ",
+              "set, unable to download file. To enable automatic model ",
+              "downloading, set url to models to environment variable
+              `ABAQUS_DOWNLOAD_URL`")
+    end
+    url = joinpath(env["ABAQUS_DOWNLOAD_URL"], model_name)
+    @debug("Downloading model $model_name to $fn")
+    dryrun || download(url, fn)
+    return fn
+end