Feature request: memory check before run

[nylander]

Add function for check min memory requirements after loading data.

Similar to the Check_Memory_Amount function in PhyML (in https://github.com/stephaneguindon/phyml/blob/master/src/utilities.c). This function will halt the run if the required memory is above some threshold, and wait for user confirmation. Importantly, Stephane also provides an argument, --no_memory_check, to turn off the checking.