Remove trailing slash in mpsd_release input

Removes the trailing slash in mpsd_release if present, instead of throwing an error and exiting. We do write a warning that this is happening:

❯ mpsd-software install dev-23a/ foss2021a-mpi
[14:01:04] WARNING  Removed trailing slash from release: dev-23a                                 mpsd_software.py:1266
Release dev-23a is prepared in /home/karnada/Sandbox/Projects/mpsd-software-environments/dev-23a
Release dev-23a is prepared in /home/karnada/Sandbox/Projects/mpsd-software-environments/dev-23a
##### Setting up spack
Cloning into 'spack'...

Closes #22 (closed)