Function Changes

[Hazboun6]

This PR includes a number of changes:

• Made the PX and SINI parameters explicitly not rescaled. The SINI default setting is Uniform(0,1) and the PX default setting checks to see if the lower bound is negative. If it is, then the lower bound is set to zero.
• The checking of nan of inf was moved from the tm_delay function to the timing_block function.
• Some functionality necessitated the need for providing the enterprise.pulsar.Pulsar object to timing_block. This will be absolutely necessary if we using non-linear timing for more than one pulsar.
• The name of the signal was changed to timing_model. This is the usual timing model. linear_timing_model will denote the "special" kind of timing model.
• Added a draw from the timing model prior.
• Added functionality to make the psr.tm_params_orig dictionary added to the enterprise.pulsar.Pulsar in the timing_block function. This negates the need to add it to the enterprise software.
• Made the default width of the Uniform priors 5*sigma
• Added functionality to use the linear_timing_model when parameters are not otherwise modeled. This is the default behavior but can be turned off by setting fit_remaining_pars=False.