Relax the requirement for source and target ByteBuffer in ZstdBufferDecompressingStream

[divijvaidya]

resolves https://github.com/luben/zstd-jni/issues/252

Motivation

At https://github.com/luben/zstd-jni/blob/master/src/main/java/com/github/luben/zstd/ZstdBufferDecompressingStreamNoFinalizer.java#L47-L48 extractArray() ensures that the buffer.arrayOffset() != 0. However, that is not a strict requirement for ZstdBufferDecompressingStream since it works with the case [1] where ByteBuffer may have a non-zero position.

Change

• Remove the constraints that ensure that source & destination buffer have position at 0.
• Added some documentation to the public methods.

Testing

• Modified existing unit tests to include cases where the source and target buffers don't have position starting from 0.
• ./sbt clean compile test is successful

[luben]

Thanks for the contribution