ximea's `get_exposure_maximum` returns nonsense

[carandraug]

With xiAPI version 4.21.19, the Camera.get_exposure_maximum method is broken, for at least the xiC cameras.

On our "Model:MC023MG-SY-UB SN:CAMAU2018044 FwF1:01.25 API:V4.21.18.00 ", the following code

``
from ximea import xiapi

c = xiapi.Camera()
c.open_device()
print('maximum: ', c.get_exposure_maximum())

reports a max exposure of of  `7.411626765319505e-38` microseconds (with version 4.21.18 it correctly reports `30000000.0`).

It seems to work properly with our xiQ cameras so this brokenness seems to be model dependent.

This has been reported to ximea, it's their internal ticket number 56174.