add int64 encoding

[crotwell]

Consider adding

64-bit integer (two’s complement), little endian byte order

as an additional "primitive" data encoding. Unlikely this would ever be needed for digitized data from an actual sensor, but since most computers are 64bit it may be useful to support this as a type. The resulting file size would be 2x, but writing an array of native 64bit integers would be possible without having to check to see if any value exceeds 32bits, for example from a numpy int_. array.

20 appears to be the next unused encoding.

[crotwell]

6 may be better choice, keep primitive types together and believe 6-9 are unused in old seed/miniseed2.