Update Camera.cpp to fix local norm for pushbroom cameras (old PR#4214) (#4340)

* Update Camera.cpp to fix not correctly calculating the local norm

This fixes Camera.cpp not correctly calculating the local norm for pushbroom cameras.
This is in reference to Issue #4018, whereas DBL_MIN is not operating as expected, causing an error later in the code, when integer rounding occurs. This erroneously returns the next/previous framelet. Currently, the DBL_MIN is not operating as expected. The functionality that is intended is:
Calculate normal by grabbing elevation values at the edges of the pixel in question.

Originally the fix was submitted via PR#4214 for branch 3.10.2 but since that branch is no longer active, changes will be submitted to dev.

* Updated Camera.h with comments addressing fix for issue#4018