example.templatetags.splangotags: on ExperimentNode init, stripped the variant names, so the variants can be declared with or without spaces

i.e. now it can be declare like this::
{% experiment "example" variants "one,two,three" %}

or like this::
{% experiment "example" variants "one, two,  three" %}
example.templates.sample: added some spaces
in experiment variants declaration.

Now spaces are not relevant.