Fix gas input for existing heat gas boilers

[koen-vg]

Changes proposed in this Pull Request

There is a potential when existing heating is added where the nodes for existing heating are only those with any CHP capacities, but when a new link is added in add_heating_capacities_installed_before_baseyear, it uses spatial.gas.nodes as bus0, which (when gas is spatially resolved) has a gas node for every node in the network, not just those with CHP capacities. This leads to an indexing error.

This PR is a simple fix for the problem. Feel free to modify if there's a better way, stylistically speaking, to address the problem.

Checklist

• I tested my contribution locally and it seems to work fine.
• Code and workflow changes are sufficiently documented.
• Changed dependencies are added to envs/environment.yaml.
• Changes in configuration options are added in all of config.default.yaml.
• Changes in configuration options are also documented in doc/configtables/*.csv.
• A release note doc/release_notes.rst is added.