Default to flat trend

[cyrilou242]

default the timseries generator to a flat trend

[cyrilou242]

not relevant anymore with the implementation of the algebra of timeseries

maybe work on the transtyping.
eg (java like) something like my_timeseries = (Signal) 3 should be equivalent to my_timeseries = FlatTrend(3)

or implement a default signal of zeroes