Make generate work with relative file paths

[mimischi]

Fixes #120.

Changes made in this pull request:

• Fixed a bug, where running mdbenchmark generate --name ../md.tpr would copy the file md.tpr to the wrong destination folder.
• Fixed a bug where specifying mdbenchmark generate --name md.namd would not parse the file name correctly.

PR Checklist

• Added changelog fragment in ./changelog/ (more information)?
• Issue raised/referenced?

[codecov]

Codecov Report

Merging #124 into develop will increase coverage by 0.03%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##           develop     #124      +/-   ##
===========================================
+ Coverage    96.12%   96.15%   +0.03%     
===========================================
  Files           14       14              
  Lines          723      729       +6     
===========================================
+ Hits           695      701       +6     
  Misses          28       28

Impacted Files	Coverage Δ
mdbenchmark/mdengines/gromacs.py	100% <100%> (ø)	⬆️
mdbenchmark/mdengines/utils.py	100% <100%> (ø)	⬆️
mdbenchmark/generate.py	100% <100%> (ø)	⬆️
mdbenchmark/mdengines/namd.py	100% <100%> (ø)	⬆️

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