improve ATF error handling

[swharden]

Improve the error message displayed when you accidentally load an ABF into the ATF module. Currently the output is very confusing.

pyabf.ATF("demo.abf")

Traceback (most recent call last):
  File "c:\Users\scott\Documents\GitHub\pyABF/src\pyabf\atf.py", line 39, in __init__
    signature, file_version = fh.readline().rstrip().split()
  File "C:\Users\scott\Anaconda3\lib\encodings\cp1252.py", line 23, in decode
    return codecs.charmap_decode(input,self.errors,decoding_table)[0]
UnicodeDecodeError: 'charmap' codec can't decode byte 0x8d in position 49: character maps to <undefined>

[swharden]

cf82e07