Refactor unet.py to remove hard-coded elements

[hvgazula]

nobrainer/nobrainer/models/unet.py

Line 41 in ed0d609

n_base_filters = 16

and

nobrainer/nobrainer/models/unet.py

Lines 27 to 39 in ed0d609

	conv_kwds = {
	"kernel_size": (3, 3, 3),
	"activation": None,
	"padding": "same",
	# 'kernel_regularizer': tf.keras.regularizers.l2(0.001),
	}
	conv_transpose_kwds = {
	"kernel_size": (2, 2, 2),
	"strides": 2,
	"padding": "same",
	# 'kernel_regularizer': tf.keras.regularizers.l2(0.001),
	}

[hvgazula]

upon further thinking, it is probably best to leave the hard-coded elements as is because the idea was to present the originally published models as is rather than make them configurable/customizable.

[satra]

i would leave the defaults to reflect original models but no reason one can't play with some of the architecture.