Add exception specifications and attributes to the required portion of code in FITS module

[gopi487krishna]

Currently the FITS codebase does not have exception specifications for various member functions in FITS module. Providing them would not only improve the documentation associated with the code but would also provide safety guarantees to the various portions of FITS code base.

Note : Kindly refrain from using Dynamic Exception Specification as they have already been deprecated since C++11 http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/p0003r0.html

Major portions of code also require attribute specification for better compiler diagnostics ( and to promote good practices in using FITS code ) . Both C++11 and C++14 attributes ( standard ones ) are allowed.

More information on attribute-specifiers
https://en.cppreference.com/w/cpp/language/attributes
https://www.geeksforgeeks.org/attributes-in-c/

Solving this issue can probably help you understand the codebase a little faster!!