fix poct header decoding error handling and acquisition date conversion

[T0mSIlver]

Fixes discussed in https://github.com/marksgraham/OCT-Converter/issues/139 Two fixes :

• Error handling when parsing an Optovue poct : construct raises a StringError when parsing an Optovue poct but the current behavior is to expect a UnicodeDecodeError.

• The acquisition date conversion is redundant. It seems to me that acquisition_date in SeriesMeta is already a datetime object, therefore the conversion in write_opt_dicom raises an error. I may be missing something because this fix is basically just reverting this previous commit https://github.com/marksgraham/OCT-Converter/commit/cbd5f00587d0b800a6057b52180a2373e75db6af. I was only able to test this on Optovue .OCT files.

[marksgraham]

Thanks for this, looks good to me!