Method to find number of cycles OS dependant

[Samuel316]

NuGridPy/nugridpy/mesa.py

Lines 915 to 918 in 1f21ffa

	cmd=os.popen('wc '+sldir+'/'+slaname)
	cmd_out=cmd.readline()
	cnum_cycles=cmd_out.split()[0]
	num_cycles=int(cnum_cycles) - 6

It would be easy to replace this with pure python code, but It only seems to be used as an in input to:

NuGridPy/nugridpy/mesa.py

Line 3725 in 1f21ffa

def _read_mesafile(filename,data_rows=0,only='all'):

as data_rows which is 0 by default?

Some of these private functions could probably be merged. At some point the whole file is read into memory for the plot so why is it read in separately to find its length?