Enhancement to errorbar() plot

Combination of plot() and errorbar_plus() can solve many prolems with the original errorbar() and can provide some more useful flexibilities. With errorbar_plus, horizontal plot of errors is possible. Different markers can be used instead of the usual "I " shaped bar. The wrong annotation of legend() when plotting with 2 or more data sets can be avoided with appropriate code arrangement.

Users are welcomed to add some other controls such as line style to errorbar.

Usage and example:
- see comments in errorbar_plus.m file, or
- download into matlab directory and type "help errorbar_plus"