Add a domain size warning when using fluid data

[mar-kni]

The method 'read_IB2d_fluid' sets the domain (envir.L). If the user initializes a rectangular domain size before 'read_IB2d_fluid' is called, then the user domain size is rewritten.

For example, if user sets Lx= 10 and Ly =10 and the IB2d simulation domain is 3x3, then the user expects a 10x10 domain but once 'read_IB2d_fluid' is called, envir.L[0] returns 3 and envir.L[1] returns 3. In this instance if the user defined domain dimensions differently from envir.L after 'read_IB2d_fluid' is called, then Planktos should return a warning.