add writing sha checksums for each output when finalising

[rpreen]

Resolves #38

Upon calling finalise() a checksums folder is created within the chosen output folder and a sha256 checksum is written for each file in the output folder.

Example output:

MY_OUTPUTS/

XandY.jfif
output_0_0.csv
output_10_0.csv
output_10_1.csv
output_11_0.csv
output_11_1.csv
output_3_0.csv
output_5_0.csv
output_6_0.csv
output_7_0.csv
output_8_0.csv
output_8_1.csv
output_8_2.csv
output_9_0.csv
output_9_1.csv
output_9_2.csv
pivot_table_0.csv
results.json

MY_OUTPUTS/checksums/

XandY.jfif.txt
output_0_0.csv.txt
output_10_0.csv.txt
output_10_1.csv.txt
output_11_0.csv.txt
output_11_1.csv.txt
output_3_0.csv.txt
output_5_0.csv.txt
output_6_0.csv.txt
output_7_0.csv.txt
output_8_0.csv.txt
output_8_1.csv.txt
output_8_2.csv.txt
output_9_0.csv.txt
output_9_1.csv.txt
output_9_2.csv.txt
pivot_table_0.csv.txt
results.json.txt