Apply minor fixes to GenericFrontend code

[SimBe195]

This fixes some minor bugs in the GenericFrontendV1 class.

1. Permit even pool kernel sizes (also discussed in Even kernel size for pooling layers in the GenericFrontend not permitted #50). Odd kernel sizes were only a problem for convolution modules and even pool kernel sizes like 2 have worked fine in testing.
2. conv_stride is supposed to be a tuple so the default value of 1 if model_cfg.conv_strides is None is wrong.
3. If pool_stride is set to None, then the expression pool_stride[1] or pool_kernel_size[1] results in an error. pool_stride[1] should only be accessed if pool_stride is not None.