This PR adds two more time functions to timeseries2velocity.py: 

+ exp: A series of exponential function(s). Modeling the amplitude of post-seismic relaxation with input:
   - list of str, dates of the onset of exp function(s), e.g., '20161231', '20190125'.
   - list of float, exp characteristic time(s) in days, e.g., 80.5, 200.

+ log: A series of logarithmic function(s). Modeling the amplitude of post-seismic relaxation with input:
   - list of str, dates of the onset of log function(s), e.g., '20181231', '20200125'.
   - list of float, log characteristic time(s) in days, e.g., 60.2, 120.

+ obects.stack.get_design_matrix4time_func():
   - add get_design_matrix4exp_func()
   - add get_design_matrix4log_func()

+ timeseries2velocity:
   - add --exp onset_time char_time option to ingest exponential function parameters
   - add --log onset_time char_time option to ingestt logarithmic function parameters
   - write periodic phase data as the following dataset names in the unit of radian to file:
       - annualPase
       - semiAnnualPhase
       - periodY{period}Phase
   - rename the periodic amplitude dataset name from using "Amp" as suffix to using "Amplitude" as suffix
Co-authored-by: Zhang Yunjun <yunjunzgeo@gmail.com>