add function to get brain entropy version

[Edouard2laire]

This adds the function to read the VERSION.txt file to get the version number of the toolbox (added here: multifunkim/best-brainstorm#8)

When looking at the installed plugin, shouldn't this number appear instead of 'Github-master' ?
edit: Done here 6c3017b

The following code

>> PluginDescription  = bst_plugin('GetInstalled', 'brainentropy');
>> PluginDescription.GetVersionFcn()

return

'2.7.3'

Note: this is ready for review :)

[rcassani]

The version in the PlugDesc comes from the plugin.mat file which is supposed to be filled by GetVersionFcn at installation. But if for some reason it is empty, GetVersionFcn is now used (for loaded or unloaded plugins).

Let me know if you want to add something else to the PR.
Otherwise, it looks good and ready to be merged!

[Edouard2laire]

looks good to me. Thanks :)