#68 Fixed unlet bug regarding deleting scale vector of current plot

[Jim Monte]

The check was not comparing the name in a case-insensitive manner, so unlet Yes would delete the default yes scale vector, for example. The code was also made more modular and descriptive regarding the check for the scale vector. Also, the warning now prints the scale vector as it is stored to make clear that vector names are not case sensitive.

[Jim Monte]

This code can be found in branch beta_jdm in commit 309b50d3a4ea4e53c73d161b7722a46903ea62a7.