Set SAM-EM default salvage value to 10%

[softwareengineerprogrammer]

Currently set to 50 (probably an oversight during initial implementation):

GEOPHIRES-X/src/geophires_x/sam_economics/Generic_400_MWe/Generic_400_MWe_singleowner.json

Line 198 in 7a197c5

"salvage_percentage" : 50,

SAM docs suggest default of 10, which is probably more appropriate:

GEOPHIRES-X/src/geophires_sam_economics/Singleowner.html.txt

Lines 716 to 726 in 9c33608

	salvage_percentage
	Net pre-tax cash salvage value [%]
	Constraints: MIN=0,MAX=100
	Required: False. Automatically set to 10 if not assigned explicitly or loaded from defaults.
	Type:
	float

Quick manual testing suggests this change would probably only have a minor negative effect on IRR/NPV in most cases (<1%)