ASIStage: keep the sign of CRISP values, limit Calibration Gain to int16

[bls337]

CRISP::GetValue skipped every non-digit before atof, so it dropped the minus sign along with the ":A " prefix: ":A -5 " read as 5. Values read through GetValue that can be negative:

• GetOffset() and the Lock Offset property (LK Z?)
• Dither Error on firmware 9.2o and later (LK Y?)
• Calibration Gain (LR X?)

Now, when a '-' sits directly before the first digit, it keeps the minus sign.

The controller stores "Calibration Gain" as a signed 16-bit value, the property now has limits of INT16_MIN..INT16_MAX.

Tested on an MS-2000.