Better error notify when ROI is too small than one pixel

[HowcanoeWang]

Discussed in #39

^{Originally posted by Ken-Kuroki August 19, 2022}
Hi, I encountered an error with certain DSM files. When I run

roi = easyidp.ROI("roi.shp")
roi.get_z_from_dsm("dsm.tif")

Then Exception has occurred: UnboundLocalError (note: full exception trace is shown but execution is paused at: _run_module_as_main) local variable 'imarray_out' referenced before assignment at the 109 line of cvtools.py.

It looks when dim==1 the variable isn't set. I eventually figured out the DSM file resolution was not enough and with small plots in my .shp file, probably each plot only made up an area smaller than one pixel. The error was solved after increasing the DSM image size.

I don't think this is an issue, but just sharing since other users may find the same error.