Fix .throw_error() and .throw_warning()

[mcol]

There are two problems with the current implementation of .throw_error() and .throw_warning() which were noted in #152:

> .validate_positive_scalar(iris)
Error: [.validate_positive_scalar()] 'iris' must be a positive scalar[iris()] 'iris' must be a positive scalar

1. The name of the caller function is not computed correctly if the caller has arguments, which messes up the output
2. If the caller is an internal function, the internal name would be exposed to the user, rather than the name of the function the user called

This PR makes the function print the name of the calling function just once and allows using them from other internal functions, thanks to the new nframe argument that allows to control which function name is reported from the error/warning message.

When .throw_error()/.throw_warning() are called from an exported Luminescence function, one can use these two functions without worrying about nframe, they will work by default as expected. The only time that we should set nframe = 2 is when we use the throwing functions from an internal function, such as in .validate_positive_scalar().

[RLumSK]

Nicely done, thanks!