Ensure that plugin entry points are always exported on macOS and Linux (otherwise, plugins won't load in macOS release builds)

The Makefile of the JuliaProcessor plugin used
absolute file paths, which prevented compilation.
I have replaced the absolute path with a shell
macro as suggested by Julia's embedding
documentation.

Fixing this revealed a second problem: the linker
flags were incorrect. I have replaced the
offedning flag '-ljulia' with build flags
generated by julia, as per Julia's embedding docs.

See issue #80

* Fixed OpenEphysLib files;
* RHD2000Thread class some refactoring and stylystic fixes.

this should work as very simple filter for one channel. The global var is shared between channels atm so it behaves weirdly on multi-chaannel data

very early alpha  -  this still has a lot of hardcoded paths etc.!