Safe vector access

[schoetbi]

Our users are not always careful when accessing vectors either for reading or even more dangerous for writing values. Therefore, I would like to restrict the accesses to vectors to be within valid bounds.

Since this costs some time to check at runtime I enable this function by defining exprtk_enable_vector_runtime_checks. When this define is not set, the old (unsecure) behavior is retained.

Runtime penalty:

Please let me know if I can improve things.