TKAP-1340: Ampacity set to 0 when heat balance is positive at 0

[johannewiborg]

• When computing the conductor ampacity, instead of getting an error if the heat balance is positive at current = 0 A, the ampacity is then set to 0 A.
• If the solution is above max_ampacity, then an exceptioin is still thrown
• The default max_ampacity has been increased to 8000 instead of 5000