ScalarEq: Implement general slope limiter methods

Reuses the implementation from SRHD.

The following parameters are added/updated to the ScalarEq section:

• hrsc (updated): added option slope_limiter
• slope_limiter_method (new): similar to SRHD.slope_limiter_method
• slope_limiter_method_tvb_M (new): similar to SRHD.slope_limiter_method_tvb_M