Adds unit tests and updates GHA

[tomaroberts]

Helps with spirit of #13

Adds unit tests, adds GHA to run on PR, adds coverage report and badges.

[github-actions]

Coverage Report

File	Stmts	Miss	Cover	Missing
nii2dcm
__main__.py	34	34	0%	5–68
dcm.py	93	2	98%	165–166
dcm_writer.py	33	6	82%	89–95
nii.py	35	2	94%	34–35
run.py	30	2	93%	57, 66
utils.py	7	7	0%	8–36
nii2dcm/modules
mr_image.py	59	2	97%	39–40
TOTAL	431	55	87%

Tests	Skipped	Failures	Errors	Time
18	0 💤	0 ❌	0 🔥	4.920s ⏱️

[tomaroberts]

Working on unit-tests branch.

Reminder to self to change to main before PR merge.