Major upgrade of the synchrotron radiation calculation code SPECTRA.

Since the first release in 2001, the synchrotron radiation calculation code SPECTRA has been used by many users and continuously upgraded to answer the requests and feedbacks from the users. Although such an incremental upgrade made in nearly two decades has significantly enhanced the capability of SPECTRA, the source code has become much more complicated, and thus the maintenance and further improvement are rather troublesome. To solve this issue, and to take advantage of the recent progress in the programming language, a major upgrade has been made as presented in this paper, in which a number of new features and advanced functions are implemented as well.