Move +bids/private stuff to +bids/+internal?

[apjanke]

What do you think of moving the package-private functions in +bids/private to a new +bids/+internal package?

This would allow these functions to be directly tested and played with during development, but would still indicate to users of the library that they're for BIDS-MATLAB's internal use and shouldn't be called from client code. This "internal" package name is a pretty well-established convention these days. I've seen it in both Java and Matlab code, including MathWorks' own code.

This also removes the need to do the extra addpath step under Octave to work around its handling of private directories inside packages. This PR updates the .travis.yml file to reflect that.

[robertoostenveld]

it is indeed used a lot in MATLAB toolboxes. I am fine with this.

[Remi-Gau]

I will defer to people with more experience on this (AKA @robertoostenveld) so I would be in favor.

[gllmflndn]

The issue in Octave with private directories inside +packages has now been fixed and will be available in the next release.

Changing private to +internal is a switch from a hard constraint to a soft one on who can call the function. I initially preferred private as the rule is then explicit and enforced but it is true that +internal will allow to write more fine-grained unit tests and will allow these functions to be called throughout the +bids package (e.g. from +bids/fcn1.m and +bids/+util/fcn2.m so that's a 👍 for me too.

[Remi-Gau]

Will merge this one as well in 24 hours.

Speak now or forever... send a PR later.