shorten! sets wrong maxlag

[tclements]

When using shorten! to subset a CorrData, if the newlag value is not a multiple of the sampling rate, the new maxlag should be set to the nearest common multiple of the sampling rate that is less than newlag e.g. C.maxlag = newlag - (newlag % (1/C.fs)).

MWE:

julia> C = load_corr(corrfile,comp);

julia> C.fs
40.0

julia> C.maxlag
20.0

julia> newlag = 16.01
16.01

julia> newlag % (1/C.fs) # newlag is off sampling period by 0.01 seconds
0.010000000000000675

julia> shorten!(C,newlag)

julia> C.maxlag
16.01

julia> C.maxlag == newlag 
true

In this example, the true value for C.maxlag should be 16.0.