small changes in get_chi and fit_routine, now xdata is typeless in get_chi and AbstractVector in fit_routine

Added methods for read_meson and read_meson_correnction that take a Vector{Tuple{String,String}} as Gamma structure. This method allow to read in one sweep all correlator for any specific gamma structure combination. It is, in a way, complementary to what was already available when one of the two gamma structure was not specified

added keyword parameter 'replica' to corr_obs(cdata::CData,...) for consinstency with corr_obs(cdata::Array{CData,1}...)

pvalue function for uncorrelated data now accept a W vector. default value is Vector{Float64}(), and if W is not provided, it is computed as the inverse of the error squared of the data.

Added new versions of plat_av. now the function accept a optional vector or a matrix of weight. wpm is still an optional paramter for backward compatibily.