Support multispectral geotiffs

[HowcanoeWang]

e.g. NDVI geotiff with two layers, first is ndvi, second is alpha

Multi-GeoTiff, with several layers, each layer has -32767 or has alpha layer

logic:

input parameter: has_alpha_layer or has_alpha_value

if has_alpha_layer: merge masks
if has_alpha_value: extract mask and merge

[HowcanoeWang]

Multiple-band test files explanation

NDVI 2 layers

Multi-spectral 2 layers

[HowcanoeWang]

GeoTiff.polygon_math not support multi-spectral

File ~/Documents/Github/EasyIDP/easyidp/geotiff.py:817, in GeoTiff.polygon_math(self, polygon_hv, is_geo, kernel)
    inside_value = imarray[mask, :]   # (mxn-o, 4)
else:
-->    raise IndexError("Only support (m,n) dsm and (m,n,4) RGBA dom")
def _get_idx(group, thresh, compare="<="):
    if thresh.shape == ():  # single value

IndexError: Only support (m,n) dsm and (m,n,4) RGBA dom