Enhance apply_features function for more efficiency

[SandeepThokala]

https://github.com/PoonLab/covizu/blob/4ab1ba45b919a45d974c0dc58bf64f09c7dfadb1/covizu/utils/seq_utils.py#L104 The overall memory usage of the list will be higher compared to just storing the characters in a continuous string. The difference in memory usage could be significant, especially when we have a large number of single-character strings stored in a list.

[ArtPoon]

Is this function covered by the unit test suite? Please feel free to optimize but make sure that the results are the same.

[ArtPoon]

Regarding commit 1b714b3, I think it is cleaner to generate a new string rather than re-use the old variable name and replace the previous string @SandeepThokala

[ArtPoon]

@SandeepThokala can you please post some timing results (with old and new code) on processing either the unit test, or a large set of data in case the test fixture is processed to quickly for meaningful timing results?

[GopiGugan]

@GopiGugan to provide @SandeepThokala with a larger dataset to generate the timing results

[ArtPoon]

@SandeepThokala can you please report timing and RAM results here?

[SandeepThokala]

Using sys.getsizeof() function to get memory occupied by the result object

	len(refseq) = 29903		len(refseq) = 100000
	old code	new code	old code	new code
time taken	0.005 secs	0.001 secs	0.001 secs	0.01 secs
memory	239288 bytes	29952 bytes	8000056 bytes	1000049 bytes

[ArtPoon]

Thanks @SandeepThokala, go ahead and merge your changes into the dev branch please