removed count in favour of invocation

[matthias-Q]

Closes #140

This PR removed the count node in favor of the more generic `invocation.

I have a question:
We have _expression, term and _select_expression (which is term or all_fields). I may have forgotten this, but why do we have these? They look almost identical and offer only another layer in the AST hierarchy.

[DerekStride]

We have _expression, term and _select_expression (which is term or all_fields). I may have forgotten this, but why do we have these? They look almost identical and offer only another layer in the AST hierarchy.

I think these have converged recently. We could remove _select_expression and just use an explicit choice rule or see if it's possible to add term to _expression.

[matthias-Q]

Now that I think a little bit about it, I think _select_expression is fine, since it combines termandall_fields`, which is used multiple times.

term adds the optional alias and _expression is also fine, since it collects all the different variations (columns, subqueries etc)