WDRT - Missing contours functionalities

[cmichelenstrofer]

There are some functions in WDRT left to migrate into MHKiT.

Task: Migrate missing functionalities from WDRT Contours. I added place holder functions for these in waves.contours. This include calculating steepness curve and modifying contours appropriately, adding confidence intervals on the calculated contours using bootstrap, etc. These would need tests and examples too (e.g. the existing environmental contours example).