Test double-observer option

[jcarlis3]

In the update to version 2.0.x, we have only focused on calculating detection functions with single observers (the default in dfuncEstim), and haven't tested implementation of the double-observer functionality that was in previous package versions. These functions should be checked and tested with double-observer data:

• dfuncEstim (where the observer argument is passed)
• F.gx.estim
• F.double.obs.prob

In the meantime, I will add an error message to dfuncEstim if the user changes the observer argument from its default value of "both".

[jcarlis3]

@tmcd82070 suggested a more-formal and more-flexible implementation of double-observer data input and mark-recapture distance sampling should be on the horizon, likely warranting a version 3.0 update.