fix max potential for CCS to 0 for t < 2020

[orichters]

Purpose of this PR

Avoid the following:

scenario  variable                                                       unit      period value
NPi       Carbon Management|Storage|Maximum annual CO2 storage potential Mt CO2/yr   2005 19646.
NPi       Carbon Management|Storage|Maximum annual CO2 storage potential Mt CO2/yr   2010 19646.
NPi       Carbon Management|Storage|Maximum annual CO2 storage potential Mt CO2/yr   2015 19646.

The levels were 0 anyway, so just affects this new variable

Type of change

• Bug fix

Checklist:

• My code follows the coding etiquette
• I performed a self-review of my own code
• I explained my changes within the PR, particularly in hard-to-understand areas
• All automated model tests pass (FAIL 0 in the output of make test)

[tabeado]

Thanks a lot for fixing this so quickly!!