Takes any value and converts it to a string compliant with the FDSN webservices.
Will raise a ValueError if the value could not be converted.
>>> convert_to_string("abcd")
'abcd'
>>> convert_to_string(1)
'1'
>>> convert_to_string(1.2)
'1.2'
>>> convert_to_string(obspy.UTCDateTime(2012, 1, 2, 3, 4, 5, 666666))
'2012-01-02T03:04:05.666666'
>>> convert_to_string(True)
'true'
>>> convert_to_string(False)
'false'