val: matrix containing monthly mean data for one year
start_year: first year
nyear: number of years since start_year
leap: logical variable If TRUE (default) leap years are considered, otherwise they are not
offset: integer values. Default is 2. Number of years considered beyond the extremes in order to avoid edge errors
no_spline: logical value. If TRUE no spline interpolation is calculated and the daily value corresponds to the monthly average value. Default is FALSE.
yearly: logical value. If TRUE the result with men value per each month per each year. Default is FALSE.
@return a matrix or data frame with interpolated daily data