Do not match string prefix at end of docstring

[panicgh]

The original docstring regex removed sequences such as r""" from a docstring regardless if it appeared at the beginning or at the end of the docstring: r"""Simulator""" erroneously becomes Simulato. However, r etc. are string prefixes and do not appear at the end of a string.

Therefore, this commit changes the regex to match string prefixes only at the beginning of the docstring and not at the end.

(Closes #84)