feat: landsat example (#50)

@johanneskoester I'm trying to get this working with gcs, but I'm
hitting issues about flags from snakemake:

```console
__main__.py: error: unrecognized arguments: --storage-s3-retries
```
We are installing the latest snakemake from main. Thanks!

Signed-off-by: vsoch <vsoch@users.noreply.github.com>
Co-authored-by: vsoch <vsoch@users.noreply.github.com>

example/landsat/README.md

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+# Landsat Example
+
+Here is an example specifying to use an s3 bucket
+
+```bash
+snakemake --jobs 1 --executor googlebatch --googlebatch-region us-central1 --googlebatch-project llnl-flux --default-storage-provider gcs --default-storage-prefix gcs://change-me --storage-gcs-project llnl-flux
+```

example/landsat/Snakefile

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+import os
+
+storage:
+        provider="gcs",
+        max_requests_per_second= None
+
+rule all:
+    input:
+        "landsat-data.txt.bz2",
+        "testdir"
+
+rule copy:
+    input:
+        storage.gcs("gcs://gcp-public-data-landsat/LC08/01/001/003/LC08_L1GT_001003_20170430_20170501_01_RT/LC08_L1GT_001003_20170430_20170501_01_RT_MTL.txt"),
+    output:
+        "landsat-data.txt"
+    resources:
+        mem_mb=100
+    run:
+        # we could test volume size like this but it is currently unclear what f1-micro instances provide as boot disk size
+        #stats = os.statvfs('.')
+        #volume_gib = stats.f_bsize * stats.f_blocks / 1.074e9
+        #assert volume_gib > 90
+        shell("cp {input} {output}")
+
+rule pack:
+    input:
+        "landsat-data.txt"
+    output:
+        "landsat-data.txt.bz2"
+    conda:
+        "env.yml"
+    log:
+        "logs/pack.log"
+    shell:
+        "bzip2 -c {input} > {output}; echo successful > {log}"
+
+
+rule directory:
+    output:
+        directory("testdir")
+    log:
+        "logs/directory.log"
+    shell:
+        "mkdir -p {output}; touch {output}/test.txt"

example/landsat/config.json

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+{"message": "hahaha"}

example/landsat/env.yml

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+channels:
+  - conda-forge
+dependencies:
+  - bzip2