rs_subset should accept multiple file inputs

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Similar to #187

With the introduction of the batching feature extraction (#148), the inputs to rs_subset is likely going to be multiple `.tiles` files. My current workaround to combine all of these files prior to running `rs_subset`

```python
import os
import csv
import argparse
from random import shuffle

def main():
parser = argparse.ArgumentParser()
parser.add_argument('dir', type=str)
parser.add_argument('out', type=str)
parser.add_argument('--ext', type=str, default='tiles')
args = parser.parse_args()

os.makedirs(args.out, exist_ok=True)

all, train, validation, evaluation = [], [], [], []

# combine tile csv files
for file in os.listdir(args.dir):
if file.endswith(args.ext):
csv_file = os.path.join(args.dir, file)

print(f"processing file: {csv_file}")

with open(csv_file, newline='') as csvfile:
spamreader = csv.reader(csvfile, delimiter=' ', quotechar='|')
for row in spamreader:
all.append(row)

# shuffle
shuffle(all)

_80 = int(len(all) * 0.80)
_10 = int(len(all) * 0.10)

train = all[0:_80]
validation = all[_80: _80 + _10]
evaluation = all[_80 + _10:]

# write new csv files
for k, v in {"train.tiles": train, "validation.tiles": validation, "evaluation.tiles": evaluation}.items():
with open(os.path.join(args.out, k), 'w', newline='') as f:
wr = csv.writer(f, delimiter=' ', escapechar=' ', quoting=csv.QUOTE_NONE)
for line in v:
wr.writerow(line)

if __name__ == "__main__":
main()
```

Start by locating the rs_subset command and read the related issues #187 and #148 for the expected batching behavior. The change is done when rs_subset accepts multiple .tiles inputs and produces the train, validation, and evaluation outputs without requiring users to combine files first.

Written by the indexing model from the issue text.