Move list validation to own function

Lists are now checked via plotoptions.validate_list(). This function iterates through the list and checks if all elements are of the given type (default: str). Optionally the strings can be checked if they correspond to an existing file.
Closes #64 (closed)

added improvement label

requested review from @m.hock

assigned to @hannes.mayr

added 48 commits

• e5a4d5bd...df7f25c2 - 47 commits from branch dev
• 2b32aaef - Merge branch 'dev' into 'improve-list-validation'

Compare with previous version

approved this merge request

mentioned in commit ee7c401e

merged