Allow reserve commands to assert balances

[upstreammuse]

There could be a use case for '= \<balance>' statements in a reserve command. Let's say you want to ensure that a particular category has a certain amount each budget period, or you want to make sure a category is fully emptied before closing it. It's an easy thing to implement at any rate.

[upstreammuse]

Link the issue to create a "top up" category type here. Create the issue if it does not yet exist.

[upstreammuse]

Even with a "top up" category, is there a use case for setting a category to a fixed balance amount? It would end up looking something like this:

<date> * Transfer
  Category  = <balance>
  Owner

With this, the program could calculate the allocation amount of "Category", which would also allow the program to calculate the corresponding deallocation amount for "Owner".

Incidentally, this is similar to the general balance-based transaction entry amount that Ledger supports, which this program does not implement anywhere at this time.