Created import:node task

Creates the means for external scripts and programs to update node attributes, without having to write directly to the document files.

--

Provide the task with a NODE variable specifying the key of the node to be updated, and then one or more additional variables specifying the attributes to be changed.

For example:

```sh
rake import:node NODE=industry_burner_coal technical_lifetime=14.0
```

You may specify multiple attributes to update:

```sh
rake import:node \
  NODE=energy_power_lv_network_electricity \
  free_co2_factor=0.2 \
  availability=1.0 \
  network_capacity_available_in_mw=74803.6
```

Be sure to surround values in quotes if the value contains any spaces, or non-alphanumeric characters:

```sh
rake import:node \
  NODE=energy_power_lv_network_electricity \
  energy_balance_group='electricity network'
```