Feature request: New logslope & c1 impute_start methods #350

[Gero1999]

I duplicated this PR (from #351) so I can syncronize my main fork and keep working on other issues while this is addressed.

New PKNCA_method_start functions that could perhaps be default/standard. Feel free to give them a look and merge them if they are useful!

Closes #350

Left problems to review:

1. Desired behavior of these functions under NA (time or conc) or conc 0 circumstances. Perhaps also to be considered for Test all imputation methods for all concentrations are NA, all times are NA, all concentrations are 0 #361, as other functions are currently inconsistent. When missing start concentration if:

Condition	PKNCA_impute_method_start_logslope	PKNCA_impute_method_start_c1
All concentrations are NA	The function returns the input.	The function adds the start time with an NA conc
All times are NA	The function returns the input	The function raises an error indicating NA time
All concentrations are 0	The function returns the input.	The function adds the start time with a 0 conc

Let me know if this behaviors are the expected, if not which one you think it should be!

2. Use of pk.calc.c0.method.c1 and the need to assert time. Check this conversation