cleanup method signatures

[lobis]

Since the codebase for this repository is still small I think its a good opportunity to do define some guidelines regarding style that could also be applied on other REST-for-Physics repositories. It may be useful to new contributors such as @AlvaroEzq or @Anakintana.

• Using const on the return type is generally not recommened (I have done it on the past myself, before I learned this). See https://stackoverflow.com/questions/1579435/should-useless-type-qualifiers-on-return-types-be-used-for-clarity for more details, but basically it does nothing, it can be viewed as a hint to whoever reads the code but generally its not worth polluting the codebase for this (you could just add a comment if it was really important I guess).

• Funcion declarations (header file) do not need to have parameters marked as const, this is only required in the definition (source file). In general it is recommended to only mark them const in the definition. See https://stackoverflow.com/questions/38660537/why-is-const-unnecessary-in-function-declarations-in-header-files-for-parameters for more details.