SwVolCube1: Make MINSTRIKE a regular parameter

The minimum strike for which volatility quotes are considered is hardcoded as 1bp currently. I observe problems in the SABR calibration when the forward is only slightly above 2%, so the effective strike of the leftmost quote only slightly above 0%. The calibration seems much more stable if one cuts off all quotes with effective strikes below e.g. 20bp. This PR allows to do so via an additional parameter in the constructor (which is defaulted to the same value as the macro held before, so this does not change anything in existing code).